Driving test tomorrow, again (C+E)

ROG:

ETS:
I’m talking about “indicating in good time”, before or after checking your mirror.

Please be more specific such as giving a scenario

I’m driving in a city, intensive traffic both lanes, I’m in the left lane. Approaching a junction where I know I will be turning right. I indicate right, look at the right mirror to see what the flow of traffic in the right lane is like and wait for an opening in the right lane. Most of the time the first 2-3-5 vehicles behind me will try to overtake me because they see my intention to change lanes and no one wants to drive behind a lorry but eventually a good soul will let me merge to the right.

My point is, how is the information you’re going to obtain by checking your mirror before indicating going to impact your decision to indicate at this very moment or not?