There are a couple of ways to check. First, measure your dominant playing hand (forehand) from the second crease down the palm to the tip of your ring finger (in as straight a line as possible). That number is most likely a good fit for you. Secondly, you could hold the racquet in a normal grip, then take the index finger of your (not someone else’s) other hand and stick in between your fingers and thumb. If there is just a little pressure needed to get it in there, the size is probably OK. If you have to force it in, it’s most likely too small. If there is a gap between them, you may be using too large of a grip.
The second method is not as accurate as the first, in my experience.