You can go to the auto parts store and buy a AC recharge can with a gauge if you want.
The current sale at napa has them for $25 for a 14-16oz can with a gauge. It may not be the same price in your state but likely similar. If you want to take the chance and spend a tiny bit of money I'd do that.
It won't tell you the weight of the refrigerant in the system but if it's low the pressure will be down. Typical low side pressure is around 45-50psi. If yours is down around say 30psi then it's down several ounces of R134a.
