What do I do if I suspect my dad is cheating on my mom?

why…will first I just wanna say..that I am so confused…I think I am so sure but not sure at the same time…well when I used to live in P.R my dad used to work in N.Y because they weren’t any good jobs for my dad in P.R and I always thought to myself what if he finds another woman and I said not my dad…than a couple years pass by and I am getting older and start to notice things…my dad gives my mom such stupid excuses like “oh I am playing poker with my friends to night and I am going to drink so I’m just gonna sleep over my friends house” or “sorry honey I am going to stay over my sisters tonight”… but one time I had my dads cell when I went to school because he lent it to me and it rang it was a private call I pick up and say “hello” she says “hello” and than hangs up I thought “ just a wrong number…than a couple hours later shes calls again and hangs up …than I was like ok that’s wierd…but I just let it pass by…than a couple months later I used my dads new cell and he had a a few contacts in it already so he lent it to me again to take to school and than I see a number call not saved just a number I picked it up and when I said “hello” no answer just a quick hang up…and than a couple hours laster I saw he made a few calls to only one specific number the number that the girl called(I think)…and that’s when I freaked out but stupid me never said a word..I just thought to myself my dad wouldn’t do a thing like that…but now that I am 18 I am putting the pieces together and I really think hes cheating on her…w.t.f do I do it likes 6:14 in the morning and I am stressing and oh I even confronted him and said “dad you cheating on mom” he laughed and said “ stop being so silly” and he never acts like that and today another fight I mean a bad one…my dad was suppose to be home at like 9:30pm and didn’t get home till like 1:00 in the morning(and she was suppose to pick him up at the train station) so my mom gets worried and goes to the train station to she if he’s there and when my mom left like 20mins later my dad comes in and than when I told dad moms not here hes like yea right shes up stairs I was like no she went to look for you at the train station..he put a face like oh crap than he called her and said…my car broke down or something… than when she came back I overheard her say…I saw you cussing at some guy…and she just said rot..and went upstairs. so yea this just happen like a couple hours ago and ugh I just don’t no what to do…please some one help me I am going crazy…?