How do doctors find out if you have a bad liver or have stomach ulcers..?

ok so i used to tak a lot of advil or ibuprofen when iwas depressed... i do not need any criticism i realized it was stupid, but i think it had a permanent affect on me. how do i find out if i have something wrong with me, ever since the last time i havent felt myself i get stomach aches more frequently.. i am a person who is never sick, i even had perfect attendence for school 4 years in a row , but now i have been getting sick a lot lately.. what to do?