The latest ones look like a standard sedan, with no visible external antennas.
The DF antenna system uses a set of slot antennas built into the roof. The
only distinguishing characteristic is the radio equipment and displays in front
for the driver, and even those have some sort of cover for when they are
not in use.
So knowing what they look like won't help you detect them, unless you can
memorize all the license plates.
The sedan type was the Mobile Automatic Direction Finding (MADF) car. Those vehicles and the equipment in them are now and have been obsolete. The vehicles were de-commissioned several years ago.
A small number (a very small number) of the vehicles were transferred to another government law enforcement agency now under the Department of Homeland Security. The rest of the vehicles had the electronic equipment removed and disposed of in whatever manner determined by the office in possession. Usually the internal equipment was sold by government auction as excess property.
Some of the vehicles had the annular slot antennas cut from the roof of the vehicles and were also transferred to another government agency for upgrades with new receivers, computer controls, and retrofit into vehicles owned by that government agency. The upgrade and retrofit was performed by an outside business.
The old "sedan" MADF cars were replaced by the Mobile Digital Direction Finding (MDDF) vehicles. The DF'ing electronics are entirely computer controlled with GPS mapping. The most current model is the MDDF 8. I believe there are now four different antennas permanently mounted in these vehicles. All FCC field offices will be have the MDDF 8 by next year if not already in possession of the vehicle.