I've never had to ask women out. I consider myself confident, but this is proving otherwise. Last Sunday I went to a fast food place with my straight sister. The cashier and I had the connection (gaydar) and she didn't stop starring to the pint of making me blush. When she brought the food to us I looked at her and she blushed. My sister stated that she definitely was checking me out. How do I ask her for her number without knowing for sure if she is gay or not? Thanks for the advice in advance.
Hitting on a waitress well they are at work... I don't think you can really do that without being rude. (What fast food place brings your food to you?) Though best advice I can offer is be brave, be direct and be ready to be shoot down because it happens. You don't want to just sit around hoping for them to ask you. You will simply be waiting around till you die of old age if you do that. You also don't want to try to be sneaky about it. Like you don't invite them out as friend and then try to hit on them. Be up front about your intention to date them romantically. Also when they turn you down, you have to be ready to move on. Not everyone is going to want to date you, but other people will. Move on and try someone new. I speak mostly out of experience doing the wrong thing here.
Instead of asking for her number, give yours instead. That way she will know this is likely romantic but you're not directly asking her sexuality.