How do I know if a guy is seriously interested in me?

Well - first of all, you need to know if he is available. Maybe he is just a real friendly guy and talks to everyone - but there is someone at home.

Ask the other staff about him. They should know.

I would just ask him in the future, but not sure of what to say if he says he has no girlfriend. One friend says I should say "Do you want one" LOL I am such a chicken. I won't ask the staff. It is small and word kinda travels quickly if you know what I mean. There are two people there that don't blab ... hm and a lady I know well. I don't think he has a girlfriend - I asked him about his 4th of July and what he did. He said oh I went to go see my male friend and watch the fireworks where he lives. I ask about most holidays after or wish a good holiday. He often talks of family or spending it with guy friends, but you never know. Maybe I will be less shy to ask things like that over time.
A car ran into me while I was crossing the street legally a month ago and a few times after my mom got food to go there. She told a couple people about me because I was curious to see if they acted like they cared. He kept talking and asking questions and said I hope she gets better and can visit here again soon. He rarely talks to my mom really when she goes in. I went in again a couple days ago and he made a big deal of it saying hi and kindly saying he hoped I was better ... how was I doing and such like he cared. Others said hi but did not ask me anything - about the accident or anything.
He is really friendly and does talk to people, but different to me. Maybe I am seeing what I want to see ... who knows?

If you don't take some action, then quit fretting about the situation.

You need to 1) find out if he is available and 2) if he's interested in you more than just as a customer.

How can you do that? Figure it out, girl.

Don't put that kind of pressure on yourself. Start small and simple.

First, if you find it difficult asking him directly that question, ask another waiter or waitress to see if he has a girlfriend.

If you do manage to find out about his lack of relationship status then ask him if he'd want to go out for a cup of coffee after his work shift. Not much of a social commitment there so you can learn a bit about him outside the diner context.

Your mean. I posted here for suggestions. I am not fretting. I thought someone may have been through this before and know what to ask.

Thanks for the nice reply. There are no other waiters or waitresses. Its a behind the counter type of place. Everyone hears everything lol I will try to think of a way to indirectly ask that ... It seems easier to ask him that then others ...
Thanks again
Too bad this seems like my only choice on what to do/say. Yes I thought coffee but in the context of where he works it's hard to see how I would do that

I reckon eye contact just a little longer than usual does the trick. If someone is into you they will hold your gaze....spark...the magic starts!

If he's not into you it will make him uncomfortable and if he's with someone he will probably bring them into the convo to ward off any false hopes with you.

This way there will always be reasonable doubt around the gaze so you can pretend like it didn't happen if needed and still be friend. Flirt girl!

Thanks SofiaMia12,
We have been flirting pretty well for around 6 months. Every time I leave - well like 90 percent of the time - I wave bye and gaze long, smile nicely even if a customer is there . He ALWAYS waves bye back or if busy say BYE to me quickly looking at me for a minute than back his task. The flirting makes my day but I wonder ...... He has never mentioned anyone, but I am not sure if he feels the same way towards me. I am definitely more than a customer, but he may think we can have nothing because I am older than him. I look young though and he looks older. I go there around 2-3 times a week. He works everyday. So we know each other as much as you can having say 2-3 5-10 minute interactions inside a often busy work environment. Flirting is fun, but I would like someone to do things with as well.

In that case just go for it. The worst he can say is no and at least you won't be left wondering as you are now. Just ask if he would like to go on a date somewhere, suggest a show or movie. Go for it! You never know unless you try.