As a rule-of-thumb, capacity information is encoded by air conditioning manufactures in the model number and date of manufacture info in the serial number. There are 12,000 Btus per ton of cooling, and air conditioners are sized by every ½ ton. Manufacturers often encode the approximate rating in Btus somewhere in the model number. Therefore it is often simple to scan the model number for a two-digit number that is divisible by 6 and to divide it by 12 to determine the capacity rating in tons. This number can be elusive, as it is not always exactly divisible by 6. Some manufacturer’s systems make this easier than others. To complicate matters, some manufacturers have changed their systems of encoding data over time.
Look for a data tag on exterior condenser unit.

Here are a few links that can help with finding the age and size.

http://www.buildingcenter.org/

http://www.wsrar.com/wp-content/uploads/2013/07/A-C-Data-Decoder.pdf