According to the UPC a 2" trap and drain can handle a 30 GPM intermitant flow.
Do you have an open 3/4 pipe discharching water or is it passing through a fixture such as a shower valve?
If you are using a valve then the paperwork should give you the flow rate in gpm.
If you are discharching through an open pipe then turn it on and see how many 5 gallon buckets you can fill in 1 min. A practical test would most likely give you better results in that situation anyway.
A 3" trap and drain can handle up to 50 gpm intermitant flow, according to the UPC