First sign is they act differently around you. You will notice a change in their personality whenever you are around? Perhaps they become cheerful or reserved than usual.
Secondly, they try to get close to you even when they don't have anything to say to you. This they do hoping to get your attention and maybe strike a conversation.
Another sign is if they always initiate contact with you. That maybe through asking for your number or if they are shy you will find them following you on your Instagram.
You will also find them often looking at you and when they notice you have seen them, they look away. In addition, also pay attention to the smiles.
Lastly is if they are trying to keep the conversation going with you. Maybe you were talking to them and got distracted by someone else, they will still try to get your win you back.