What types of antennas are used in a Piper Seneca PA34-200?
There can be quite a few antennas:


VHF Comm
VOR
Localizer
Glide slope
DME
ADF
Transponder
Marker beacon
GPS
ELT with integral GPS receiver
ELT
FM Receiver


You can search for antenna at a site like chiefaircraft.com to see what they look like and the frequencies they are designed for. Or go to a manufacturer website, like COMANT Industries, for the same.

Some may have multiple functions in one antenna.

Typical locations: 


Top of fuselage/wing, (VHF comm, GPS)
bottom of fuselage (belly), (transponder, marker beacon, VHF comm)
top of tail, (VOR, glide slope)