Arkham Horror LCG Talent Timing
It has to be done before drawing the chaos token. From the rules:


  Before drawing a chaos token for a skill test, the investigator may boost his or her skill value.


The rules then say there are two ways to do this; you can commit cards, and/or


  ...the investigator may activate (free trigger) abilities to modify his or her skill for a test.


It would make the game significantly easier if you could selectively boost your skills only when you know you would fail otherwise.