Yeah, so basically you’d need to go to your state’s police department and request a background check. Typically this will cost you a little bit of money, and you’ll have to send in a form or two. Otherwise, you can turn to the FBI and have them do a national criminal background check. Any convictions which have not been stricken from your record due to age will be present unless there is a law in your state hiding your offenses for a particular reason (offense is minor, you were a minor at the time, etc.). Some states will report back only felonies, some states will report back anything they consider a “crime”. You should either look into your state’s laws, or else just pay the ten(ish) bucks, send the forms in, and just see what actually shows up.
Spain will not let you in if you have been convicted of any crimes for which there is an equivalent law in Spain, and which can be considered a serious crime. Therefore, there’s no telling what they’d do for something like possession. It’s clearly illegal in Spain, but whether this would bar you from entry is completely up to the consul who reviews your application.
Also, keep in mind that when you request your criminal background check in almost all states you will need to also request that the background check be notarized. This is normally done for free upon request, but may vary depending on your state. The reason you need the notarization is because in almost all states notarized documents are the only ones eligible for an apostille, and an apostille is required for all legal documents you submit in your application.
