You have to identify geographic coordinate system of your data first.
Go to feature class properties and in XY Coordinate System tab you will find this information.
Here's an example:
[ATTACH=CONFIG]16117[/ATTACH]
Identified geographic coordinate system can be found in:
Coordinate Systems/Geographic Coordinate Systems folder.
for example: WGS 1984 is in World subfolder, NAD 1983 is in North America subfolder.