By the shape of their tails & head.
The female has a founded tail, the male has a longer, pointed tail.
The male has a broader head.
You cant always tell 100% by this as they do vary a bit depending on their locale.
i have a pet shingle that i think is a female because i found another one a few weeks ago and it was very agresive and it is spring where males do tend to get very agresive so i was guessing it was a male but now i have found another one that is very plascid and kinda and is not agresive at all so im thinking it is a female? is it or is it not a female?