Most Helpful Guy
The fact that you have so little trust in him, that you want to make a fake snapchat account to find out if he's cheating again is a recipe for disaster in this relationship! Even if he was faithful and loyal to you enough to avoid flirting with you on this fake account, who's to say you wouldn't try something similar to this again if you feel like he was cheating on you again later on?
This won't solve anything as the damage has been done and building up trust again requires trust on both your end and his. If it's clear that his behavior changes such as him avoiding you, spending a lot less time with you than he usually does and/or he becomes less flirty around you, it's time to have that serious conversation about it. If he continues to ignore you, then it's time to move on. It's never healthy to be in a toxic relationship where you're constantly worried about being cheated on all the time.
Most Helpful Girl
Obviously the first sensible reason is that if you don’t trust him he’s not making you feel very secure in your relationship so you’d just be better off leaving. BUT I can understand why this would also be a good idea, at least this way you’ll KNOW for sure and never wonder if you was right or not (because you’ll likely be confused by his past behaviour since you decided to stay). This will give you closure and you’ll gain a sense of confidence in trusting your gut instinct which clearly I think you need to be honest to give you the courage to go.