hi i have a skink and i wasnt told its sex when i bought him how would you go about telling ?

If they're anything like blue tongues, you can't really sex them unless you see sperm plugs or you try to breed them.
Feeling the base of the tail might give you a reasonable guess though, soft for male and hard to female, but it isn't exactly reliable because of fat stores there.
Still, it turned out to be right for my little guy.

__________________
0.1 Carolina Corn Snake (Cookie)
1.0 Blue Tongued Skink (Till)
0.2 Dogs (Ami -mix- Nuka -northern inuit-)

im not expert but it does appear to be humping the glass

__________________
1 Fire Salamander (S.S. Bernardezi) (Chalimander)
3 Chinese Fire Belly Newts (Hoysin Herbert Minute)
4 Mandarin Salamander (Morphing) (The Fantastic
1 Golden Apple Snails (George) (RIP Sharky)
4 West African Land Snails (Limicoloria Flammea)
4 Golden Land Snails (No Names As Of Yet)