How Do I Know If I Need A 15 Or 20 Amp GFCI?

How do I know if I have a 15 or 20 amp circuit?

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Should I get a 15 or 20 amp GFCI outlet?

Is it OK to put a 15 amp outlet on a 20 amp circuit?

What is the difference between 15A and 20A GFCI?