Some are very obvious, others not so much. You really just have to look them over and then decide whether or not to worry about it. We have a lot of scrappers in our collections, depending on the pin we just don't worry about it.
Please do NOT follow this advice, this is bad advice as these pins are almost 100% sure to be scrappers.
We don't trade much at night either, unless it's a pin we just have to have. We actually have small flashlights we carry on our lanyards so we can see the pins in better lighting at night. I've been burned soooooooo many times, not so much by CM's but by regular people just trading. It's quite frustrating to know that the pin you thought was in good condition is actually quite beat up.
Even buying 200-300 at a time is not a guarantee of good pins, in fact, quite the opposite IME. Before I knew about scrappers I would buy lots like that, huge quantities for trading...and then someone told me about scrappers and I realized the pins I'd been trading (and getting) were probably all fake. So I made a concerted effort to educate myself and honestly, any seller with that many pins is a HUGE red flag for me.
Anyone interested in buying pins for better prices please follow the link in my signature. It's quite helpful, and yes, quite large but it's a good place to start.