I would drop a hit on a piece of paper and taste part of where the drop hit. If it tastes just like paper you know that it is either LSD or nothing at all which rules out RCs. If there is no funky taste then go ahead and eat the whole tab and wait an hour or more for effects. You'll know if it is LSD if there is a reasonable amount in your drop. If possible, wait with your dealer for the effects to kick in. That way you will more than likely purchase the vial that your hit came from. There are shady people out there who might attempt to switch vials with something either weaker or bunk just to make some extra cash, so its better to narrow down the chances of such a thing happening by waiting with your dealer.