I usually measure the can, on the opposite side to the fan.
But I will also try to get a measurement through the vent holes periodically to make sure that the inside is no more than around 10degC hotter than the can.
This way I know I'm extracting a lot of heat.
If there's a large difference in temp, the heat is getting trapped inside and no matter how powerful the fan you use, it's still going to build up where you don't want it.
Xray T4'16, T4'14 (Wet Car)
Xray X12 2016
wlrc.co.uk (West London Racing Centre)