Totally agree with IaP, Kat.
Unless you're full of insecurities, then I'd say follow your gut instincts. Has your partner ever given you reason to doubt him before? Have you discussed your worries with him?
If he's cheating, I wouldn't say it's automatic that you ditch him, but it shows that there's something wrong with the relationship which needs sorting out quickly, if it's to survive.
If the guy's NOT cheating, then you have to ask yourself why you're so suspicious, and if you're over-reacting.
Oh - and if you're almost 100% sure he is - do as red says, and follow him!! Best of!