There's usually some sexual dimorphism in reptiles - bigger, bulkier male lizards, crocs and some turtles; bigger female snakes and other turtles; colours; spikes and dewlaps; femoral pores; concave plastrons etc. - but some types are more overt than others and if you need to be sure it's handy to have a gander at the tackle. Problem is they're usually tucked away, as Beastlord says. In lizards and snakes you can see the bulge behind the cloaca, or try to evert them; although another common method (with snakes, at least) is to use specially-manufactured probes to determine the length of the inverted internal 'pockets'. You can buy sets of these in exotic pet circles, in accompanying containers or wallets like a set of wax carvers from Heresy.
It's a similar method for crocs, though with different results, and the 'probe' is a little more personal. The external features of turtles are usually enough to go by.
I'd post links, but it's possible to go too far with the mental scarring.
In terms of the dragon, it's whatever Andy says it is.
And a 'vague line' is good enough for a reptilian creature.
Also: oooooh pics.