Your question seems a bit confused. In your second question:
For example is it after the trilogue or after Parliament and Council votes?
You appear to be assuming that is has some kind of formal say or role in the EU decision making. It does not, except in an advisory capacity to Ministers, who have statutory authority as they take part in EU decision making.
The European Scrutiny Select Committee is part of the House of Commons. Its Wikipedia entry, while lacking details, offers the helpful summary that follows, to which I've taken the liberty to add [some comments in brackets]:
The remit of the Committee is to assess the legal and political importance of each EU document, and decide which EU documents are debated [in the Committee]. The committee also monitors the activities of UK Ministers in the Council [there are Minister related Councils in addition to the better known European Council], and keeps legal, procedural and institutional developments in the EU under review [that is, they're a watchdog group and can bring up problems when they think something's up].
Steve added this helpful link that is a bit more specific on its advisory role:
Debates recommended by the Committee take place either in a European Committee or (more rarely) on the Floor of the House. Under the scrutiny reserve resolution passed by the House, Ministers should not vote in the Council of Ministers on proposals which the Committee has not cleared or which are awaiting debate. Where a proposal has been debated in European Committee, the motion must then be put to the House, the following day where it is agreed to without debate. A proposal is cleared when the motion is agreed to by the House.