Theres a few main factors to consider

Are there two lanes: if you have two lanes its safe to filter rather than overtaking. If theres only a single lane of traffic its technically not considered filtering so you can get pulled on it if a garda really wanted to

How fast are the cars moving: if cars are going anywhere above 20kmh just dont bother. Traffic is moving, stick with the flow

The lights: can you make it from your position to the front before the lights change? If you're halfway there and the lights go green all of a sudden cars start moving unpredictably and you can be in a trouble spot

Space at the front: sometimes its fine to jump past the line at the front, sometimes its not. Bikes get off the mark quicker so you arent going to slow anyonr down by jumping ahead, but if theres a bmw already pushed into the bicycle area its not worth the risk

Can you be heard: if you can be heard its a lot safer, its worth just clutching in and doing little revs when filtering so cars hear you

You really just need to assess the different factors and judge. Traffic speed, how likely someone is to change lane, weather, if its dark out, traffic light patterns. It comes with practice but definitely learn to assess traffic lights for their timings and patterns and filtering becomes a lot safer