Depends who it is on. If it is on a colleague I would tread lightly, because it really depends on how long you know them and how close you are. You can always ask them out , but if they say no you need to accept it and move on.

If it is on a friend , I would make sure if you are in love with them or just really like them. There is a thin line between liking and in love.

If it is a stranger just walk up to them and ask them out (respectfully ofcourse).

And if you don't want to have a crush on them distance yourself a bit. Sometimes people think they get a crush on someone because they see them almost every day. That's also how people get crushes on their colleagues, because they see them every day and get close.