This is a really interesting question. It depends on what you're trusting them about. I think it's really naive to say that you would trust anyone 100%- sure, there are a lot of people that I would trust to be generally safe people to be around (not hit me, assault me or emotionally abuse me), manage money competently, be reliable when they say they'll do something etc etc. With those things, you can go off actual track records to judge the likelihood of a person being trustworthy in the future. If, for example, I've been with a man for 20 years and he's never been violent or coercive towards me, it seems reasonable to feel confident that he's unlikely to become violent in the next 20 years.
Trust in someone's sexual fidelity is a totally different matter because (in my opinion) it's extremely common to have learnt to be extremely secretive about aspects of your sexuality that you think others would disapprove of. That's fine, of course- everyone is entitled to sexual privacy. But using my earlier example, I can only say that I THINK the man I've been with for, say, 20 years, has been faithful. While I can know whether my partner has abused me, been unreliable about feeding my cat or forgotten to turn up for important events, I can't possibly know if he's cheated. Plus- The internet makes it more possible than ever for people to have secret sexual lives without anyone having an inkling.
Of course, women also cheat, but I know of far more couples who broke down because the male partner was unfaithful. I think most people are capable of infidelity with the right stressors. But I've had male friends (in the past, no longer) that openly admitted that it was the novelty of sexual variety that made cheating attractive, not any particular unhappiness in their relationships. To complicate matters, they were all generally reliable in most other ways, so it's not even that sexual untrustworthiness necessarily comes as a package with other forms of untrustworthiness.
So, while there are many men I'd trust for many things, I wouldn't trust any of them 100% to be faithful. That doesn't mean that I'm hyper vigilant about the possibility- I've sort of just made peace with it as an occupational hazard of intimate relationships, with anyone but men in particular, and always try to maintain the practical and emotional resources to manage if things go tits up.
I wouldn't say anything, but I'd be scared for any of my friends that seriously suggested they trusted their partners '100%'- I think that makes you really vulnerable, as far as I'm concerned everyone should have a proper plan for what they'd do if their spouse (or any partner they're financially connected to) fucked them over.