Yeah, women will make it obvious when they like you or they will flat out tell you. The flat out telling part generally comes when you aren't showing interest in them already. And you also need to not be oblivious to when a girl is making it obvious to you.
If they go out of their way to talk to you, get your attention, sit by you, touch you, anything like that it could be a sign of interest. If they are overly nice to you, not just a general nice, but like an out of the way nice they probably like you (though this might not be 100% sexual attraction). An obvious one though is when you catch them taking glimpses at you when they think you aren't paying attention or aware that they're eye fucking you. First you might want to ask yourself do I have fucking mayonnaise on my face or something, and if not, they're probably checking you out.
If they're trying to feed you, or make sure you eat, it's flirting. It's not direct, but it displays they are thinking about you and care about you. If they're asking you personal questions, unprompted, this can also be a sign of interest. "Where do you live? Are you married? Do you have kids? Where do you work?" that type of shit.
There's also physical seduction, or attention seeking type things they might do. Bending over in front of you, wearing skimpy clothes around you, basically trying to show themselves off to you. This is a lot harder to pick up on, and I generally wouldn't take this as a sign without other signs.
Why all of this happens and why people cheat can be for a lot of different reasons.