It is quite easy to balance the carbs on a '81 XS650 without any special balancer. However it is not done by checking whether the butterflies are moving the same, but by checking whether the cylinders are running the same. Because of differences between various things regarding the individual cylinders, this may actually only happen when the butterflies are not in the same position exactly.
An XS650 will tune very close by simply feeling the output of the exhausts, place your hand about 1/2" from the exhaust, and the pressure and sound must feel the same. Obviously if you have a two into one exhaust this can't be done.

To begin just start the bike, let it warm up, and adjust the idle up to about 1400 RPM using the normal idle adjustment screw. Then pull the plug lead from the RIGHT side. (It is actually better to ground the lead than pull the plug, because pulling the plug can cause damage to some electronic ignition system, but it won’t on an XS650. It can also cause carbon tracking on the coil, but again this has never happened to me in years of doing this on these bikes, so I would just do it by pulling the plug lead. In later years learn how to make a little grounding rig and ground the lead. But for now just pull the lead off the plug.)
Adjust the idle to about 700 rpm while running on the LEFT side only, using the normal idle adjustment screw. Then reconnect the plug lead, and pull the LEFT hand plug. Set the idle to the same 700 rpm, but this time use the screw in the linkage between the carburetors. Then reconnect the plug lead, the bike will idle somewhere around 1200 to 1400 rpm. Adjust the idle to your desired level using the normal adjustment screw (should be around 1200 rpm). Fine tune the balance using the screw in the linkage between the carbs, and feeling the exhaust outlet as described above. This will only adjust the running of the right hand cylinder, move it to run either faster or slower, until the sound and feel with your hand covering the exhaust feels the same for both cylinders. Then finally readjust the idle using the normal idle adjustment screw.