At you're first actual prenatal appointment they should take blood and check hormone levels. This would be the first indication of twins. Hormone levels double with a normal pregnancy and quadruple with twins. Then they will confirm with your first ultrasound.
You can find out the sex at 18-20 weeks. Most drs do it at 20 weeks though!
Im not really sure about the sex maybe at 5months?
But they would be able to tell you if your having twins or not at possibly 5weeks and older? Im new at this , just a quess this is my 2nd pregnancy.