Genetic frogs?
But to answer, I think it varies for me, mostly depending on color and sometimes eye focus. To give you an example:
In this photo, I assume the 2 in the back to be male, and then in the front row, going from left to right, male, female, male, male, female.
My reasons for the female ones are pretty vague...for the first one, I think it's due to the body and neck shape, and the second would be due to the coloring. Mouths also play a big part in my decision, for example the third frog in the front row would be female if it wasn't for mouth shape that tells me otherwise. It's weird how I associate gender with shape and color, but I do for some reason.