No, you can't rely on that. You can set the shutter speed well beyond the camera limits and it'll be limited by the camera. To measure the actual shutter speed, you'll have to use a (preferrably calibrated or otherwise reasonably accurate) device moving at a known speed, so you can calculate the shutter speed based on the speed and the travel distance recorded in the image. A commonly-used test is the Dremel-drill test, where you take a picture of a quickly-rotating dremel cutting disk. It might still be slightly inaccurate, though, since i.e. I'm getting 215 volts from the wall sockets (on average) where the device is calibrated at 230V, I think. I think it'll still be accurate enough to get a global idea of the shutter speed. Combine the measurement results with shutter speed bracketing, where you take several images using a faster and faster shutter speed and then analyze their histograms to see which shutter speed does not make a difference anymore. You'll find these two values to be pretty close together and there is your shutter speed.
