First of all, you need to make sure that your fish is fully grown. You’ll find it really difficult to find out the sex of your pleco if he or she isn’t fully developed yet.
If your fish is an adult, you should be able to tell its gender by doing the following:
Check the shape of its forehead. Males tend to have flat, smooth foreheads whereas females usually have rounded, bulging foreheads.
Look at your fish from above. If it’s slightly rounded, or if it looks like your stomach does after Christmas dinner, then it’s probably female. They tend to be bigger and wider than the males.
In some species of pleco, only the males have spines at the sides of the mouth. If your fish doesn’t have the spines, it’s female. That doesn’t mean that your plecostomus can’t be female and have the spines, though – it all depends on what species it is!