You don't have a thick skull!You are a trainee, you are asking because you don't know, this is normal, and it is the right thing to do. Those with the thick skulls have all been killed or injured because they didn't find out and guessed wrong.
Anyway you asked so i will give you the correct way.
Identify circuit to be isolated.
Switch off circuit (Trip breaker/pull fuse).
Fit a suitable isolation device and lock off
Using an approved voltage indicator and proving unit.
Prove voltage indicator with proving unit, it should light up.
Using voltage indicator test each pole to every other.
3 steps if single phase L-N, L-E, N-E
6 steps if 3 phase no neutral L1-L2, L1-L3, L2-L3, L1-E, L2-E, L3-E.
10 steps if 3 phase and neutral. Same as 6 step plus these 4.L1-N, L2-N. L3-N and N-E.
Voltage indicator should not light.
Reprove voltage indicator with proving unit it should light up.
You have now completed "Safe Isolation" to the standard required under the Electricity at Work Regs.
A known live supply can be used if you don't have proving unit.
If you don't have a voltage indicator and proving unit, then buy one.
You cannot use a multimeter of any kind to carry out safe isolation for 2 reasons. It is not the correct instrument because it is not a Voltage indicator, and it can be selected to the wrong range or setting, which can lead to interpretation of wrong results and you getting it all up your arms.
if you don't safely isolate properly, you can and will get prosecuted if you get a shock and live to tell the tale. You will be asked some awkward questions from people who you would rather not be interviewed by, the local A&E wont think you are clever, and you certainly wont be laughing about it in the pub, but the local undertaker may thank your family for the business....
It isn't rocket science but it needs to be done right.
Enjoy.