The simple technique to measure a solid body bar's rail spread is put a small straight edge against the side of the bar and the chains cutting edge working corner.
If there is no clearance between the straight edge and bar then it's time to tighten up the rails, and that means a chain filed down to the witness marks also.
I always hammer my bar rails on an anvil with a shim to tighten them up, also good for getting bends out . To get bends out of a laminated bar that the center shim is not broken [do a bend test in a bench vise] is to hammer the bar on an anvil but with a piece of belting between the bar an anvil.
Laminated bars rails can't be successfully tightened like a solid body bar can, they loosen up quick again with just a little use.
I like to file my cutters right down to the witness marks until the cutters break off so a on spec rail gap is important to me. Very important with a saw that fells trees, with the b/c laying on its side and the chain filed small the operator can't afford any extra gap.