Drive your car onto level ground (like a parking lot), and then find your measurement spot. Then repeat when you're back in the driveway making it level. That's what I've done, actually using a magnetic level, shims, and tape.

I expect professionals do the ATF level check when the vehicle is on a lift, so the 4 lift points define what "level" is.

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2001 X5 3.0i, 203k miles, AT, owned since 2014