How do I know if an outlet found on the outside of a garage is GFI?

Thanks

This site shows a GFI outlet and a GFI breaker

Installing, troubleshooting and testing GFCI Ground Fault Circuit Interrupters From The Natural Handyman Home Repair and Do It Yourself Website (http://www.naturalhandyman.com/iip/infelectrical/infgfi.html#1)

Both have a button to test and need to be reset if they trip

This site probably has more than you care to know

A GFCI outlet is quite recognizable with its test and reset button in the middle and the raised rectangular area around where the plugs go. How ever, regular outlets may be protected by being fed from the load side of a GFCI elsewhere. Code requires them to be marked. Yeah, right. Often they use the same distinctive rectangular plug area. You could track down any CFCI outlets and push their test button. If that kills an outlet, it is fed from the GFCI and is GFCI protected. Beyond that, I guess you could carefully set up a high resistance path to ground, through a light bulb or something and see if it kills it. Note, doing so will leave it off until you track down the GFCI feeding it and reset it.

If you Have a wiggy(voltage tester) plug into hot and neutral, should read voltage.
Now insert wiggy into hot and ground, If it is a receptacle downline of a GFI, it will trip.
You will now have to locate the GFI that tripped, Or GFI breaker that tripped.
Another way would be, verify voltage at receptacle, Then turn off any and all GFI breakers,
Then trip all GFI's In house. If you still have voltage, likely not protected.
You can install GFI receptacle for protection.