Some of the typical standards used are "free and fair multi-party elections" and the existence of the "rule of law". Wikipedia reviews a variety of definitions of the entry on "democracy" stating (citations omitted), that:
Although democracy is generally understood to be defined by voting, no
consensus exists on a precise definition of democracy. Karl Popper
says that the "classical" view of democracy is simply, "in brief, the
theory that democracy is the rule of the people, and that the people
have a right to rule." . . .
Democratic principles are reflected in all eligible citizens being
equal before the law and having equal access to legislative
processes. For example, in a representative democracy, every vote
has equal weight, no unreasonable restrictions can apply to anyone
seeking to become a representative, and the freedom of its
eligible citizens is secured by legitimised rights and liberties which
are typically protected by a constitution.
One theory holds that democracy requires three fundamental principles:
upward control (sovereignty residing at the lowest levels of authority), political equality, and social norms by which individuals
and institutions only consider acceptable acts that reflect the first
two principles of upward control and political equality. Legal
equality, political freedom and rule of law are often identified as
foundational characteristics for a well-functioning democracy.
The term "democracy" is sometimes used as shorthand for liberal
democracy, which is a variant of representative democracy that may
include elements such as political pluralism; equality before the
law; the right to petition elected officials for redress of
grievances; due process; civil liberties; human rights; and elements
of civil society outside the government. Roger Scruton argued that
democracy alone cannot provide personal and political freedom unless the institutions of civil society are also present.
In some countries, notably in the United Kingdom which originated the
Westminster system, the dominant principle is that of parliamentary
sovereignty, while maintaining judicial independence. In India,
parliamentary sovereignty is subject to the Constitution of India
which includes judicial review. . . .
There are many decision-making methods used in democracies, but
majority rule is the dominant form. Without compensation, like legal protections of individual or group rights, political minorities
can be oppressed by the "tyranny of the majority". Majority rule is a
competitive approach, opposed to consensus democracy, creating the
need that elections, and generally deliberation, are substantively and
procedurally "fair," i.e. just and equitable. In some countries,
freedom of political expression, freedom of speech, freedom of the press, and internet democracy are considered important to ensure that
voters are well informed, enabling them to vote according to their own
It has also been suggested that a basic feature of democracy is the
capacity of all voters to participate freely and fully in the life of
their society. With its emphasis on notions of social contract and
the collective will of all the voters, democracy can also be
characterised as a form of political collectivism because it is
defined as a form of government in which all eligible citizens have an
equal say in lawmaking.
Republics, though often associated with democracy because of the
shared principle of rule by consent of the governed, are not
necessarily democracies, as republicanism does not specify how the
people are to rule. Classically the term "republic" encompassed both
democracies and aristocracies. In a modern sense the republican form
of government is a form of government without monarch. Because of this
democracies can be republics or constitutional monarchies, such as the
Western and liberal democratic system political observers do not generally consider one party states to be fully democratic, even though they are Republics, because they do not formally recognize a hereditary monarch, even if they are not democracies. This is because citizens are not given a meaningful opportunity to exercise choices about the nature of the government, and who represents them in it, in a "free" manner.
The same link also discusses how this is operationalized:
Ranking of the degree of democracy are published by several
organisations according to their own various definitions of the term
and relying on different types of data:
The V-Dem Institute's Varieties of Democracy Report is published each
year since 2014 by the Swedish research institute V-Dem. It includes
separate indices measuring five different types of democracy:
electoral democracy, liberal democracy, participatory democracy,
deliberative democracy, and egalitarian democracy.
The Democracy Index, published by the UK-based Economist Intelligence
Unit, is an assessment of countries' democracy. Countries are rated to
be either Full Democracies, Flawed Democracies, Hybrid Regimes, or
Authoritarian regimes. Full democracies, flawed democracies, and
hybrid regimes are considered to be democracies, and the authoritarian
states are considered to be dictatorial or oligarchic. The index is
based on 60 indicators grouped in five different categories.
The U.S.-based Polity data series is a widely used data series in
political science research. It contains coded annual information on
regime authority characteristics and transitions for all independent
states with greater than 500,000 total population and covers the years
1800–2006. Polity's conclusions about a state's level of democracy are
based on an evaluation of that state's elections for competitiveness,
openness and level of participation. The Polity work is sponsored by
the Political Instability Task Force (PITF) which is funded by the
U.S. Central Intelligence Agency. However, the views expressed in the
reports are the authors' alone and do not represent the views of the
MaxRange, a dataset defining level of democracy and institutional
structure(regime-type) on a 100-graded scale where every value
represents a unique regime type. Values are sorted from 1–100 based on
level of democracy and political accountability. MaxRange defines the
value corresponding to all states and every month from 1789 to 2015
and updating. MaxRange is created and developed by Max Range, and is
now associated with the university of Halmstad, Sweden.
Other indices measuring freedom and human rights include degree of
democracy as an element. Some of these include the Freedom in the
World ranking, the Worldwide Press Freedom Index, the Index of Freedom
in the World, and the CIRI Human Rights Data Project.