I find it's easiest to just mention it to the marshalls. Don't approach them all fire and brimstone, screaming about how the bad man isn't taking his hits. Just let a marshall know and say "I'm not sure, but could you keep an eye on [insert description of potential cheater here], as I think they may not be taking hits."
But if I think someone isn't taking their hits, I'll always mention it to staff and then leave it to them to enforce. I don't see the negative stigma of it, unless it's every game day, but if that's the case it's probably the shooter not hitting their target; I've seen so many airsofters have a moan about hit taking, but their BBs are falling short because their gun fires to 60 airsoft metres, but only 40 actual metres.