There is no way for the Scottish Parliament to call a legally binding referendum
Referendums are not legally binding. Under any circumstance, the UK parliament would have to approve the breakup of the Union. The UK has a tradition of "government by consent" (as least a relates to home nations) In the event of a clearly and consistently expressed will of the Scottish people to leave the UK, it is unlikely that the Government would prevent it.
A comment mentions the 1776 solution: i.e. violent revolution. However, a more directly relevant comparison would be 1861. If an SNP government were to attempt to become independent by violent means they would have to overcome a much larger force... This scenario does not seem remotely possible at this point.
(Couple of notes: An act of parliament can have the effect of making a referendum binding, however such an act can also be repealed. It would have to be a Westminster bill, since it affects the whole of the UK.
The note about government by consent. See how the Irish uprising of 1916 didn't lead to all out war. Or even how, post-1945, the British Empire was dismantled. While this wasn't always the case, at the end of the day, the UK government doesn't often attempt to maintain control of territories against the will of the population by force.)