Why don't you get into the habit of being present when the aircraft are being refuelled, that way you will be able to correlate visual indications with fuel volumes required to top off the tanks.
Anyway why is it a problem to get the aircraft fuelled if you are not sure?.
At the end of the day it's your licence and future on the line if it goes wrong. The CAA will not accept the argument that the CFI said it was OK.
If you are new to the game, the CFI should be encouraging a safe approach until you are happy with a quick look that tells YOU its OK.
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TwinComm