I have a 6" Reliant jointer and it is/was slightly not co planer, I could slide a piece of paper under the in feed side a bout half the length of the in feed table. I helped it a bit but could not remove it entirely by adding shims between the table an the rail at the end of it.
My 1st and only jointer, inexpensive and not well made but it works well enough.
If you don't have a reliable long straight edge, something you might try is to keep pressure only on the infeed table run an item 2/3 through, shut the jointer off, keep pressure on the item infeed side then see if you can slide a sheet of paper, step flashing or feeler gauge under the far end of the item.
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