Ok here's what you look for when spotting conversion games.
First you want to know if its a game that would be worth converting, this means usually that both the US and JPN versions are over 500 dollars. Games like Metal Slug 1, Big Tournament Golf/Neo Turf Masters, Ninja Masters, Pulstar, and a few others are fairly commonly converted carts.
MVS only games are also always conversions, games like Strikers 1945 Plus, Prehistoric Isle 2, and Bust a Move.
Usually but not always conversions will not have a manual, if it doesn't have a manual and is one of the games that fits the criteria above, most likely a conversion.
After that you actually have to have the game in your hand, and do comparisons between the print quality of the art.
Hope this helps, most people on this forums (well-known, established members) will always tell you if its a conversion, otherwise use caution and common sense on eBay.