I have been working with plecos for some time. While body shape can help. it is not anywhere near 100% reliable. If I have two plecos in a small tank which are close to sexually mature, I can easily determine their sex by observing their behavior. But I work with plecos on groups. When they are not close to spawning age.size, the only way I know to reliably sex them is by venting them.
If you can have them put their tube out, you can tell from the shape of it what sex they are. The male will have a more pointed triangular shape while the female will have a more rounded blob shape, he only needs to pass out sperm, but she need to pass eggs.
Here are pics of two male and two female Hypancistrus L173b
Male
Male
Female
Sorry this one is less clear, but you can still just see it.
Here is an interesting fdact about plecos gals and eggs. When I would discover a dad on a newly fertilized batch of eggs I could not figure out how they could have fit inside the female. The egg pile was too big to have come from such a small fish. It took a more experienced fish keeper to explain it to me. When the eggs are inside the female they are somewhat dehydrated and that means they are smaller. As she lays them they immdeiately absorb water and swell to their full size.
There are other physical clues which can be helpful in sexing if the fish is not too young. males tende to get bushy odontile growth near the gill plate. They will also often develop odontile growth along their flanks. These serve as a weapon when males spar to determine a pecking order, and thus, who will get to spawn and who will not< Just as the males in a gorup have a pecking order , so too do the females. Only the top females get to spawn. This also means that the bigger females, who can make/hold the most eggs, are the ones who will normally get to spawn. It is just another example of how mother nature works to insure the odds of the best genes getting passed on and the most offspring being produced per spawn.