When you say test the mcb, do you mean check if it's operating at the correct current, or that it allows corrent to pass through it ?
If the mcb is lying on the bench, switch it to the 'on' position and use a continuity tester or low resistance ohm meter to check for continuity between the bottom, where it (usually) connects to the busbar,and the top, where the outgoing circuit is normally connected.
If the circuit breaker is installed in the board/ consumer unit,(assuming the main switch is on) and the busbar is live. Using an approved voltage indicator, measure between the top of the circuit breaker and the neutral bar. You should read the supply voltage. Simples !!
Testing the cuircuit breaker for correct operation is a little more complicated.
You will need the operating characteristics for that particular breaker,an adjustable load of some sort and a clamp ammeter. Best of luck !!
Speedster.