How do you know when someone is their age?

I approach as often as I can but I have had experiences on the past where the woman I was talking to was below my age standard(under 25). I always ask if they recently graduated college, usually as flirty line but also to see if they are under 25.

There are looks that I can discern from one woman to another but as a whole what signs should I look for to only approach women above 25?