Basically it would be the same as when an younger woman likes you but there's usually some hesitation. It can be more uncomfortable for an older woman to show interest in a younger man but sometimes it's hardly noticeable.
She will usually look for reasons to 'bump' into you, she'll smile, and laugh, she'll enjoy your company and listen carefully when you speak.
She may innocently brush up against you.
If you're sitting beside her she may lean into you or cross her legs in your direction.
She may reach over while she's talking to casually touch you.
More confident older women may even make suggestive comments or hint that they like younger guys.
She may look for signs that you like her. For example she may ask you if you like her perfume or something she's wearing. This is personal and a signal that she might be interested.
It's usually very obvious that an older woman likes you but sometimes you can be fooled.
If there's a huge age difference be careful. You might feel that the possibility of falling in love with an older woman is remote and therefore getting involved with her is emotionally 'safe', but if a relationship develops it can be just as hard to break it off.
Also some women are just naturally friendly or they just like to flirt. They have no intentions of dating you or 'going to the next level'. Most women are much harder read than men because women tend to be more soft and subtle. You don't want to get into a situation where you make a fool out of yourself thinking that she's got the hots for you when she really doesn't.
One thing you can do is observe how she is with other men. Does she flirt with everyone or just you?
You can also joke about how much fun it would be to date an attractive older woman or how you find older women attractive.
You might say something like 'Soooo ..., I heard that you like young guys. Is that true?' Smile and laugh. Keep it light and subtle.
If you come on too strong she may get scared and reject you. The best thing to do is throw your fishing line out there and see if she takes the bait.
If you talk about dating or romance in a lighthearted non serious way it's hard to get rejected. She'll never be sure if you're serious unless you tip her off.
Your fun approach will force her to do or say something that will let you know she's interested or not.
If she's not interested you can admit to being a big flirt or all talk and no action, etc...
If she is interested, the transition to romance will be much more comfortable when you don't make a big production about the age difference.