If your showerhead is relatively new, the gallons-per-minute consumption rating is inscribed somewhere on the outside of the fixture. If it's not, do this quick test.

Hold a 1-gallon container under a faucet or fixture and time how many seconds it takes to fill the container with the faucet opened all the way. Then divide 60 by the number of seconds it took to fill the container. For example, a fixture that filled the container in 30 seconds would be rated at 2 gpm.

While you're at it, it's a good idea to clean any screens in the fixtures of any scale deposits.