The correct term is "guardian angel", thiough I'm still not sure what you mean by 'how do you know who it is'.
If guardian angels actually exist, then it is very unlikely you will ever see one, not even your own.
Having said that, possibly you are talking about another person who is said to act "as if" they were your guardian angel. Such a person might or might not choose to reveal themselves to you, and depending on how skilful they were, you might not even know that there was someone looking out for you.
In that case you'd have to "catch them in the act", as it were, doing something beneficial on your behalf.
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