If you have a smartphone, there are tons of "bubble level" apps that will tell you the angle
Just calibrate by taking a reading, then flip the phone 180 degrees and measure again. If the measurements don't agree then the app needs calibrating to re-zero.
regarding running the fridge off level
TONS of good info on that subject here
https://www.arprv.com/
many pages or write ups and even several videos where he shows the boiler temperatures when sitting at angle.
I'm not affiliated with them in any way....just sleep a bit better with the thing installed.
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