I've licked my wife's pussy several times after someone had cum in her.
Most of the times she had been out and I knew what she was doing and I licked her after she got home, so it was probably at least 30 minutes after they had last cum in her. I could still taste the cum and I'm sure I would know if she had been fucked even if it had been a couple of hours before.
Same with sex after she's been fucked by another. I've been with my wife as well as some other women and somehow it seems to feel different when it's another guy's cum rather than your own and I think I'd know it. I've played with her pussy up to about an hour and a half after she's been cummed in and she's still always had that silky slipperiness.
So yes, I'd know and until we talked about it recently she hadn't even gave it any thought when I had gone down on her after other men had had her. I think the other answered are implied from what I've written so far. she wouldn't need to tell me and I'd know as soon as I went there, if I hadn't already known what was happening... which i usually do.
Women that are freshly fucked have a look about them that is unmistakable and the smell and taste different, even when they have used a condom you can tell by the smell and taste of their skin and I love that "cat that ate the canary" look that tells you she did something very dirty, and enjoyed it.