It's difficult to say since various factors can affect sizing and them reaching sexual maturity. What I have read states waiting until they are a year old and looking at their size. Males (full size about 4.25 in) will always be a little smaller than females (about 5.25 in). There will of course be outliers, but this is apparently a good way to tell when other indicators aren't present. Some factors which can alter sizing is feeding frequency and amount, which can push a frog to reach its full size a bit faster or slower depending.