If you get beyond making them smile at the thought they are looking good, usually in dress up clothes, showing a bit of leg or cleavage, occasionally in a swimsuit that shows "just a bit more than normal", and if phothgraphy comes up, take a lot of pictures, be complimentary, after all this is about making her feel good. If you have the privacy, ask casually to show just a bit more, say non threatening things, like "opps, can't show that one in public". Let her lead. Wear thin slacks or shorts, that make is possible, but not easy, to see when you get hard. If she starts touching herself, or starts touching you, then the legs are opening.
It also works on the middle aged and older lady, who may not be sure that the "looks good" anymore, and may be really happy for the attention. Have a model release, so you know you aren't getting someone underage, and mention that you don't publish your pictures, they are for you and her alone. Sometimes a cute candid portrait will lead to an invitation to come over for a "shoot".
So, tell them they are looking good, don't be creepy, don't be pushy, at the least she will feel nice. She may decide that she wants you.
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