Count the rays in the dorsal fin. Try to get a picture with the dorsal fin fully extended. That helps to exclude certain species at least. How old do you think it is? If still a juvi, they are even harder to ID.... at least my previous rescued pleco was, even for the experts on PlecoFanatics and PlanetCatfish... but eventually it grew up to be one of the "common pleco" species.
Well it definitelyhas more than 10 rays in it's dorsal fin so Pterygoplichthys sp. looks good. Since it's still a juvi, you could browse through all the photos on the below profiles and look for juvi pictures but P. gibbiceps and P. pardalis are two "common plecos" found at pet stores... as well as a couple of other "common plecos". Here's a list of all of the Pterygoplichthys sp. listed on Planet Catfish.
Interesting thing is that the spots stay about the same size as the fish grows. So a baby like that just has a few spots. When they get to a foot long, they have hundreds.