Most PSL's are essentially AK-47's on steroids with reinforcing plates at the end of the receiver to keep from cracking during receiver bend under recoil of the 7.62x54 Rimmed cartridge. While the PSL has a bent metal style receiver the Dragunov is primarily a milled receiver.
The PSL is a long stroke gas piston system like that of the AK family, the Dragunov uses a short stroke gas piston system. Supposedly Dragunov thought this would reduce the recoil impulse on the shooter. The trigger group of the Dragunov is, from my understanding, different from the AK style of trigger group since it helps with getting a better trigger pull. Both semi-auto and select fire AK trigger groups dont easily do as well with getting a nice crisp light pull weight.
The key things to look for when on the look out for a true Dragunov or perhaps a Polish SVD (direct copy of the Russian Dragunov) are the bent safety selector, vertically halved fore-end, a gap between the trigger and magazine release lever and lack of reinforcing plates at the end of the receiver.