There can only two ways to trigger an anticipated general election
A bill was passed in 2011 about "anticipated general election".
It can only be triggered if:
A government bill is defeated, for instance if May was to put forward a bill to trigger Article 50 and that bill was rejected.
If 3/5 of parliament approved an anticipated general election. Basically the opposition would need to vote with the government. When you look at the polls, there is absolutely no way Labour would accept it.
In a nutshell, if parliement vote against triggering Article 50, then we'll have a general election.