You dont, some people just know!
Not getting a period would be a clue, but that can happen for other reasons as well.
Take a test
deny it if you havent done it
if a pregnancy test is positive, then there is a big chance that you may be pregnant. if you havent had any intercourse without protection then you shouldn't worry.
Go see a doctor and have a test done and a ultrasound.
They would not know. That is not a test that is done in a drug test.
Stress, change of life styles, it could be a many of things.
see link below "Can you have a paternity test while you are pregnant?"
Well I would say that you should take a home pregnancy test to make should that you are pregnant but I mean I dont think you have to spot to be pregnant some women spot when the baby inplants but that is like 10 days after having sex! but anyways take a test to know for sure that you are pregnant!!!
At 18 you must know how babies are made. If you have had unprotected sex, of course there's a risk for pregnancy. Take a test.
You can easily call the place where you're having it done and ask beforehand. Before that, I recommend you take a pharmacy bough pregnancy test to know if that's even an issue.
see link below "Can you have a paternity test while you are pregnant?"