PS1 was famous for its anti-piracy measures so I doubt you'd end up with a counterfeit/adulterated copy. PS1 discs are also black on the underside so that's a really hard type of disc to get these days. Seconding what Anankha said, I doubt you can edit or rewrite a genuine game disc. I believe they are all read-only and there are plenty of security measures in place in the always online realm of modern gaming to flag when that sort of thing comes up. Plus if you're suspicious you can inspect the discs for any sign of counterfeiting.
Most people don't have the technology to produce the correct style of label printing for at home jobs. Printers are pretty good these days, but you can generally tell if a disc has been home-printed. Has a distinct inkjet appearance, generally, as laser printers can't print to discs due to their process using heat to fuse toner onto the surface (bad for discs). You can get printable disc labels that you could use in a laser, but again that would be very obvious - much like someone sticking a fake photo over a dodgy piece of ID or whatnot.
Another thing to consider is the effort vs incentive vs reward for someone to maliciously tamper with a second hand copy of a game. Most scammers aren't in this kind of open ended, uncertain long-con (however it is always prudent to remain proactive and aware). No doubt you can buy a disc that's just crappy or too damaged to work though, but that's just down to neglect not necessarily anything nefarious.
