If you sort the talkgroups in number order (Group All in tables) like
Dothan Public Safety P25 Trunking System Profile
www.radioreference.com
You will see that the dispatch talkgroups end in 1 like
5251 D Dothan FD 1 Fire Dispatch
Probably the TAC and other talkgroups are numbered
5252
5253
5254
...
52??
depending on how many talkgroups they have.
If the tac channels happen to be conventional repeaters or simplex then you will have to go through all the FCC licenses and find any that aren't part of the trunked system.