Depends on what you mean by how much power you are getting. If you mean how much power are you drawing, you can purchase a wattage meter such as a Kill-a-watt. This tells you the amount you draw from the wall, not the amount that is being provided to the components, but you can guesstimate this by multiplying by the PSUs efficiency (usually advertised on the manufacturer website).
If you mean how powerful is your PSU, you can take the side off the case - the wattage and other key specs will be printed on the side of the PSU.
As PC Eye has said - don't try to directly measure the output from the PSU.