Yes, by listening, that is how we do it. When eyes and ears are on the ground in the presence of a talk group transmission, you can either wait for them to identify the channel or talk group or actually do outside and witness the unit talking, if possible.
This is why it has taken so long for me to analyze the Bell County system in Killeen, Texas. Yes, I have many talk groups, but if I am not 100% sure about who is talking and on what channel/talk group, I don't submit.
In the number of years I've been doing this, I have submitted about two that were wrong. It is rather embarrassing to do this. Park on those talk groups and listen. Eventually, you are going to hear them talk about going to another channel/talk group. Good luck.