Pubic lice (crabs) and scabies are both caused by tiny insects that inhabit hairy parts of our bodies. You'll probably know if you have them because they' incredibly itchy - particularly when they get into your pubes - and you normally start itching within 6 weeks of catching them.
Scabies, as opposed to crabs, could also cause an itchy rash, not just on and around your genitals but also on your buttocks, your abdomen, under your arms, around your nipples and on your elbows, wrists and hands. Scabies does this to you because the female mites are burying into your skin to lay their eggs and you can sometime see the red spots they cause on your skin.
You can often see crabs with the naked eye. The same goes for their eggs.
Crabs and scabies are easily passed on through all forms of close physical contact, not just full penetrative sex. Scabies can also be passed on through sharing beds or linen but it's very rare for pubic lice to spread this way.
To test for pubic lice a doctor will first and foremost try to spot the lice. Scabies are harder to diagnose, and a doctor's diagnosis will probably rely upon your history of scratching, a rash and perhaps an examination of a skin sample.
Both crabs and scabies are treated with a shampoo or lotion. Partners and family members may need to be treated too.