How do i know if my husband is cheating or not?

i am a stay at home mom to our 5month old baby. so he works at night and for the last few weeks we have had no sex, last night i was begin for it and he was giving me nothing, 5mins before he had to leave fr work we had very fast quicky. he often goes out of the house and doesn tell me where he is going and if i try to ring him he doesn't answer, alot of times he does leave his phone at home. im at breaking point, something isn right?