Hi,
If you apply a voltage step at the input of your amplifier (at t=0, for example), your output voltage will be composed of two regions. The slew rate region, and the linear settling region. The slew rate region will be the first part, because there is where the derivate of the output voltage will be highest (At t=0). This means that if you want to measure slew rate, you must measure it around t=0.
This is assuming your amplifier can be modeled as a one pole system.