How do you know when to ask girl on a date?

Been hanging out with a girl at work for a while now and each time I ask her to do something there's always an excuse relating to how busy she is. But she will sometimes ask me to help teach her something or hang out with her some time. I guess I like her but not sure if these are mix signals or if she would be on the same page as me.
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Sorry I need to be more descriptive I work oncall and see her like once a month. And when she ask for help it's more like 'hey you should help teach me how to surf some times' not related to work
How do you know when to ask girl on a date?
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