How u know if your boyfriend / baby daddy cheating on u if y’all not around each other 24/7

So I’m pregnant it’s my second baby and it’s a girl and me and my baby daddy/ boyfriend be texting every day but it’s like slow he doe text me every day to see how the baby doing and check up on me and the baby and he told me he trying to start school and work soon then I find out on Instagram he like this girl pic she was in short shorts and half shirt and her post said under her pic hard to handle I didn’t tell him anything about I find out about it but I told him way before I didn’t like it and he said he will stop liking it so I’m trying to see what I can do and say for him to understand I don’t like that and to see if he talking to somebody else or cheating without asking him because he might say he not I know how guys is they lie bad but i just know it’s something but it might just be me but give me some ideas what y’all think I should do and he 25 and I’m 23 about to be 24 December and he do come see me and the last time he saw me he always hug me and rub my stomach and everything and kiss me and I’m in my second trimester right now and I’m due February 27 2023 he says he gonna be there for me and the baby but he trying to start this big job and stuff and his phone off so he have to use WiFi and he have a iPhone but it still says deliver when I text I thought if your phone was off it don’t says deliver at all but he says it still do that and he with T-Mobile and I’m with AT&T I do be doing little heart eyes and saying love him sometimes but we really haven’t been saying that he just been calling me baby mama since he find out I was pregnant and we together and everything I just wanna know what’s the signs he might be cheating or hiding something if he not around me ? And what I can do to see if he cheating way before I got pregnant I use to make a text now app to see if he reply but um And I really haven’t did that in a long time because he says something about his ex use to do that so I’m not trying to act like her and shit