If you are using a hardware peripheral on-board the MCU to support the I2C bus as a master then there is a timing control register that controls the bus clocking speed. If a HW interface is being used as a slave responder then the bus speed is primarily set by the other end of the interface.

On the other hand if you are doing the I2C master mode interface as a software managed bit-bang interface then the timing of the bus is determined by the way the code is written and how fast it executes. You can determine the actual speed by counting instruction cycles or using an oscilloscope to measure the timing. (Note that almost always when bit-banged I2C is implemented it is a good idea to validate that the interface is operating at the intended speed using the oscilloscope).

Michael Karas