Welcome to the forum. In order to find the F generation of your bengal, you need a complete pedigree going back to all the Asian Leopard cats in the pedigree. A bengal will have several hundred different cats and multiple ALCs in the pedigree. The F generation is determined on the shortest link to the ALC. I spent 3 months doing my bengal's pedigree through the bengal pedigree website. Unless ALL the cats are listed, you won't be able to find out for sure which F generation your bengal is. When I registered my bengal with TICA, I ordered a 5-generation pedigree, which was complete on his dad's side, but I had to put in five or six cats on his mom's side, including his mother, to get Raiden in the database.
http://www.bengalpedigrees.com. You cannot determine the F generation from a photo, although most early generation bengals can appear more like the ALC, especially F1s.