MVS Scans is your best friend. As are clear cases if the seller won't open the cart up to show the boards. Most of my MVS games were bought off of ebay and none of them are boots. I tend to avoid buying from Korea and China. According to the Neo-Geo forums, France has a bad rap. Try and look at the sellers history and see if any of their sold carts were boots. MVS Scans also shows the cart label which can be a helpful indicator of what to look for. Uncovered eproms, obvious soldering work, Chinese lettering, and strange font for the SNK logo are all good indicators. I'm sure I'm missing some other bootleg traits.
I'd be happy to help you if you need.
MVS carts don't have security screws, you just need a Philips head.