Best way to measure the capacity is to time how long it takes the battery to discharge to a cutoff voltage while being discharged by a constant current.
I agree with Joe, only special SLAs will deliver their rated capacity at the 1C rate; more likely 0.1C, meaning that you use 0.7A to discharge a 7Ah battery and its terminal voltage should stay above 11.5V (or whatever the mfg specifies) for 10h.
Here is a circuit that will maintain a fixed voltage of 2.49V across R2 (hence constant current of 2.49/R2) while moving most of the dissipation into the NFET. You can change the current by changing R2. Plots show V(c) and the dissipation in R2 and the NFET as the battery discharges from 13 to 11V.