I have seen a couple of threads on this with different methods, but here is how I do mine and how I use legos to make it easier.
There are many phone aps you can use, and this process can be used to “square” the phone. I personally don’t use the phone aps, but a bubble level and cheap Home Depot inclinometer are more than sufficient for my needs. I also use some legos to create a flat “even” surface and square to measure against. My system runs about $20 and being able to expand or shrink legos means the system is “universal”.
Basics: Always measure on the playfield itself (not the glass). Adjust the casters on the feet to level the machine.
Step 1: Find 2 points on your playfield that should be the exact same difference up your playfield. I usually use the tops of the slings.
Step 2: Put down a ruler, piece of wood, or something long enough to bridge the two points chosen. I use legos because I can make them the exact size I need.
Step 3: Level the machine left to right using a buble level.
Step 4: Add a square, or if you are using legos some legos going up and down.
Step 5: Use my inclinometer to measure pitch and adjust front and back legs as necessary. 6.5 to 7 is normalish, but I usually go around 6.8. The higher the pitch, the harder and faster it is.
Step 6: Repeat steps 3 and 5 until both measurements are accurate.