Usually the callsign will be "Omaha - - -" followed by a mix (usually) of phonetic letters and numbers.
Example:
"Omaha 4 Hotel Sierra"
"Omaha 8 - 5 Uniform"
This convention is the callsign ("Omaha"), followed by the last few (2-4) characters of their tail number.
I use
this website to identify the aircraft type.
The two above are:
1. Eurocopter EC-120B Colibri, N374HS, found
here.
2. Piper PA-42 CheyenneIII, N9085U, found
here.