To catch these, the locals would walk down the street with cash in their hand sticking out. It really seemed like they were trying to hitchhike but with gas money. On the highways there were a lot of "buses" like the one in the photo below that would just pick up these cash hitchhikers as they went.
Old cars (1950's Chevrolets) "Almendrones" (old car)
Allowed to pick up foreigners since Feb. 2011
Unmarked
Recognizable by the taxi sign in the front window
Another clue, people regularly get in and out of them
Offer a one way route
usually along the Viazul bus line routes: Parque Central at the corner of Prado and Neptune to Miramar or Mariano
People stand along the route and hop in/out
Usually charge tourists in CUC
If you can speak Spanish you'll be charged in CUP
To help with not being overcharged, don't get in at the hotel or Parque Central
If you wish to take it
raise your hand when you see the car coming
don't say anything, just get in
once inside, give the street you are going not the exact address
don't ask how much
if you are going to the tunnel: CUP 10
if you are going to Miramar or Mariano: CUP 20
pay the driver as you get out of the vehicle
make sure to have exact change