I leave a small bubble level on the tt deck, always verifying level.
Bubble Levels can be defective. I have several various plastic ones, they all agree with each other (and they agree with my more professional levels I double check with).
I just bought a nicer stainless steel one with blue fluid to look good with McIntosh blue meters for an amplifier stand I will be selling on eBay. The new one is off a few degrees, what a shame, perhaps a tiny imperfection inside that prevents the bubble from centering. My quest for blue fluid is not done, the first one said blue, showed blue, came green.