Using the surface plate is definitely the way to go. You need an
external reference.

I would work the dial indicator from both surfaces. Start with the
dial base on the grinder and indicate to the plate in X and Y
directions: level the surface plate as needed so it reads zero at the
ends of the travels. This gets the surface close to the average plane
of the table motion.

Now you can put the dial base on the surface plate to indicate the
whole surface of the table. Check the table when it is in several
locations. Actually, you could remove the table and check the ways as
well.

Another test of the ways is to place a precision level on the table and
watch the bubble as the table is moved around. Check for cross-level
(twisting) as well. In this case, gravity is your external reference.

By the way Machine Tool Reconditioning has a whole chapter on grinders.
If you want to see ultra-precise, find a copy of Moore's "Foundations
of Mechanical Accuracy" where he describes how Moore jig grinders are
built and checked.

Bruce