Triggers can be tricky and slip up on you, but, if you know what the negative behavior is associated with the trigger (like eating if someone hurts your feelings), you can work backward and figure out your triggers. Make a list. When you go for the comfort, write down what triggered you to need that comfort. Before long, you will have them readily identified--once you identify them, well, that's half the battle.
It depends on the person. For me, it would be a picture of blades, cuts, or blood. If you're trying to identify triggers, try looking at whatever you suspect could be a trigger. If you want to cut/harm, look away immediately. If not, then whatever it is most likely is not a trigger.
Identifying triggers can be really difficult. I started by noting down every time I engaged in harmful behaviors, and what was happening right before them. Over time I noticed a pattern and was able to see my biggest triggers. Some of them were obvious, some were more subtle.