Sweep the indicator mounted in the spindle on the Y-axis. A 6” parallel is useful as you gain some diameter.
Note whether it’s higher in the back or the front.
Remove the headstock, revealing the dovetail plate.
Loosen the screws on the plate and insert a shim. Start with a .001” thick shim.
Depending on the condition you may have to shim the top or the bottom. You will have to realign the dovetail plate in X after this and tram again. This can make you crazy and you will probably have to do this a couple of times. Don’t try to get it perfect.
Here’s a shim peeking out from under my headstock.