The carbed models are easy to spot, they have a normal fuel tank and are marked plain F650, Funduro or Strada. The FI models have the tank under the seat so the filler is behind the rider (makes fitting luggage a pig). These are the F650 GS and CS.
I had two of the carbed models and loved both. My last one went from the North Cape to Morocco. There are however some problems. First the water pump is a disaster waiting to happen. The seals go, water and oil mix, the head warps and you need a new engine. Second, these are not real BMW's of the air head twin variety. They corrode just like the Aprillia they really are. So, dip the oil and look for rot before you buy. A blown water pump seal is a 40 quid kit and half a day job IF you catch it in time.
Why no FI? It's ultra reliable, less fiddly to fix if it did go wrong and gives better MPG. It's far easier to change a fuse than deal with a twin carb single with a blocked jet or sticky float. The electronics either work or they don't. They only usually fail if some muppet did welding without taking them off. The tank under the seat would be my only issue with a GS over a pre-2000 model.
Andy