First of all, I'd respect that yellow locking paint. Honda did much effort to have carbs which hardly need syncing. Checking - preferably without disturbing things - is OK though.
The high idle can well have another cause, like the central idle adjuster needing readjustment*, airscrews turned out too far, a non OEM airfilter element and/or airleaks. As far as bench syncing, I had success using a small allen key gauging the small opening manifold side. You need to be concentrated though and do it by feel, which means: take your time. Some use steel balls which should drop simultaneously. Making a pic of where the adjusters screws are now, wouldn't hurt. It is well possible Keihin handed the racks over to Honda with the adjuster screws all having the same postion, number of turns wise. Personally I find the light method rather inaccurate.
* Realise that the CB500 needs a raised idle when cold and the first mile or so. Some keep the throttle somewhat open, as the Owner's Manual suggests, others (like me) use that idle knob for that.
Warning: do not try to achieve a highest idle possible by turning the airscrews out. Although this may work on other motors, the result on yours will be a serious loss of driveability and possibly a hanging idle.
A standard check before turning to the carbs, is to verify the throttle cables do not hamper operation and the ignition advancer moves smoothly. It goes without saying that ignition timing is correct and plugs and plugcaps are sound.
