There are several ways.
1......You will not have a period for months.
2......Your belly will start to swell and your breast will get bigger as they fill with milk.
3......You may get urges for strange or different foods than you normally eat.
4.......Go to your doctor or if you can't afford than to a free-clinic and have a pregnancy test done or buy one at a drug store take it home and do the test yourself.