For modern fine-wire spark plugs, you should not measure the gap because you could damage the center electrode. But, it is still a good idea to inspect the new spark plugs for damage or defects. The majority of these plugs come pre-gapped from the factory.
On these types of plugs, bending the ground electrode(s) can alter the geometry and thermal performance of the electrodes to the point where it could cause a misfire.
On older applications, gapping may still be required. Gapping specifications can be found in the service information or emissions tag under the hood. Also, gapping specifications can be found in electronic and paper catalogs.
Measuring the gap requires the right tools. A coil-style gap gauge will work for copper core plugs. For fine-wire or precious metal plugs, it is recommended to use a wire-type gauge.
If it is necessary to widen the gap, do so with a tool that only pulls back on the ground electrode without touching the center electrode or the porcelain. To close the gap on a plug, gently tap the plug, electrode first, on a hard surface.