Turn the power off to the HVAC condensing unit. Do this by turning the circuit breaker off that serves the unit.
Unscrew the service panel that is located on the unit. It is typically located on a corner of the unit and is secured with four screws.
Remove the panel and look at the information diagram on the inside. It will contain all the technical information about your HVAC condensing unit.
Look for the line item that lists the BTU of the condensing unit. It should list a number between 12,000 and 60,000 and is usually in increments of 6,000. This is how many BTUs of cooling your condensing unit provides.
Divide the number listed by 12,000. This will tell you how many tons your HVAC compressor is. For example, if the BTU number listed is 36,000, divide it by 12,000 and you'll see that you have a 3 ton unit.
Screw the service panel back on to the unit and turn the circuit breaker on.