how do you know if he/she is the right one for you

its really hard to know that the person is the right one for you, but to give it a shot, you will just feel it deep inside that she/he is really the right one for you, like even if the most negative side of her or his, you will still learn to accept it.

You must get to know them over a period of timer, and don't marry someone who is a loser or who has long standing problems, life is tough enough

Let your gutt tell you. Not your heart and not your mind but the feeling you get deep down in you belly is the one you need to listen too. If you get butterflies when he/she is around or the empty feeling within when he/she isn't then it is probably love. But let that feeling be there for quite some time to know the truth.