Loosen the bolts under the table and get the mitre slot and fence set up.
Every third tooth on nearly all blades have no set, so use that for butting a ruler against.
You can use a pair of blocks against the fence, and your ruler the far side of the blade and check for a gap between.
easier than if you were to use the fence alone.
You could call it a poor mans "bandsaw buddy"
Alternatively there is two other methods.
One method is to keep ripping thin slices off the right side of a jointed bit of timber until it's right
Another is getting a combination square or pencil gauge to draw a parallel line up some sheet material
(again jointed edge)
make the cut freehand (without fence) up to about half way, and set the fence or better yet table,
(say if you made your fence parallel with slot)
so that the fence is parallel with the freehand cut.
Ps mind your head when you stand up after adjusting trunnion bolts.
Might as well check if the table is 90 to the back of the blade, say for cutting tenon shoulders
you would not want to go beyond the bassline.
Likely you could also make the leafs of the table flush if they weren't already by way of either shimming or should there be two sets of nuts inbetween table and trunnion no need to find washers.
Tom