Thank you for your question. The first things you want to look at is if he has any rings on his fingers.
Some times other things that are gifted from people that are in a relationship is a nice watch so you also can look for that and you can also comment on the watch by saying what a nice what did your girlfriend give that too you? This will answer your question if he is single.
You can also drop hints that you are single and are interested.
There are little things that will tell you if he is interested in you.
He might touch your hand or shoulder when you both are in the store of tap you in the arm when someone says something funny.
He might look into your eyes and watch your face for facial expressions. He might look down at the floor turn red in the face if he shy.
If he is interested he might ask you questions about yourself or he might try to keep you in the store longer.
You can say something like you work a lot of hours does your girlfriend mind that you work this much?
You can also tell him what you do for work to get a conversation started and also mention that you are single in a subtle way.
* suttle way
You could say something like I work a lot of hours but I do not have a boyfriend.
making suggestions is always good as well like offering to go out for coffee or maybe out for lunch.
See how he responds.
People that are interested will want to get to know you better so they might talk more, ask questions, flirt.
Someone that is shy might not look at you direct, turn red in face and be awkwardly shy. They might look uncomfortable because they like you but are so shy that they act different around you.
They could drop hints when you leave like can't wait to see you again, are overly friendly.
You also could ask someone if they know if he is single maybe a co-worker or someone that is friends of his. But it is best to ask him or ask certain questions to see if he is single.
You can say something too him like this, "Maybe we could go for coffee some times." This is simply suggesting that you are interested but no pressure.
It is a very relaxed way to ask someone out to just get to know them first. You could also say, "I feel like I know you, I'm in here so much, would you like to grab some lunch some time?" This will also let him know that you are interested.
If you do not want to ask then look for sign first like flirting, not wanting you to leave and him asking you questions. You also want to share a little bit about yourself each time. Like it's so hot out, I think I will go swimming, or going to relax after work eat dinner and watch a movie. You want to share little pieces of your life so that he begins to develop a connection with you. He will begin to get to know you. This is a step in the right direction.
Also look to see how he responds too what you are saying if he listens and wants to know more. If he is not interested he might listen but just say something simple like have a nice night.
That is someone that only is listening to a customer and not someone they are interested in.
So it is important to watch for all signs. Thank you again for your question.