What's the accuracy of the data sampling clock of LSM303D?
The datasheet doesn't say. Clearly, this device is intended for applications where the clock parameters are not important. It doesn't even have an option to use an external clock. One has to assume that they're "good enough" &mdash; on par with the other performance specifications of the device.

You can measure the frequency accuracy by simply counting the samples as they become available.

But in order to measure the clock jitter, you would have to give the sensor a known signal (i.e., put it on a shake table) and analyze the phase noise in the data stream.