What degree of scratches,dents or other deformities are acceptable for use in reloading?
What Duke said, but it also depends on caliber, and who was shooting it. Let me explain...
I load over 30 calibers, for some of those, I rarely find discarded cases. One day I thought I hit the lottery. A bunch of IDPA shooters had left a big pile of .38 Super brass. It looked good so I took it home, cleaned, inspected, and loaded it. I noticed that when I was loading it, the primers went in really easy. After loading 100 rounds, I was putting some of the completed rounds in a box and noticed that about a half dozen primers had
fallen out! The brass was spent. The primer pockets had been shot loose.
The lesson I learned was "If experienced shooters/reloaders leave the brass, there's probably a reason." In other words, if I leave any .223 cases behind at an NES shoot, don't reload them.
You also learn what to pick up and what to leave. Leave any steel or aluminum cases, anything with an AMERC headstamp, and other stuff you learn with experience. For example, I leave an 9mm with a crimped primer pocket. The reason? There's so much 9mm around that I can afford to leave the cases that require an extra step in preparation.
Also, when I buy once fired brass, how do I know that it is?
You usually can't know for sure unless you're the one who shot it.
On some calibers, there's a dead giveaway: If you find cases (.223, .308, .30-06) with crimped primers, you can be sure that it's once fired because nobody recrimps the primers when they reload.
Another clue is when you find shiny brass all with the same headstamp, and some empty factory boxes in the trash that match the headstamp.
On some calibers, it really doesn't matter. You can reload .45 ACP almost indefinitely - I have some where I can barely read the headstamp. If you find decent looking .45 ACP (or .38 Special, or any other low pressure caliber) you can be pretty sure it will load fine.