The usual dynamic test for head bearings is pretty easy to do - the steering geometry should damp undue movement, and it will if the connections between the various parts are in the ballpark, the most important of which are the head bearings. If you take your hands off the bars and coast down slightly from 60 or 70 km/h (keep them close to the grips, but not touching) and then lightly punch one grip - the forks WILL shake, and if the head bearings are correctly adjusted and in good shape, the movement will damp out very quickly ( like after two or three 'shakes' at the most) if the bearings are too loose, the shaking will be slow to damp itself , may increase in amplitude, and you will be forced to grab the bars to stop it. Sounds a bit scarier than it is, really, but it's not a good feeling exactly, either. If it fails this test, tighten the head bearings, and run the test again. The tighter the bearings, the quicker it will damp fork oscillation, up to the point where the bike has to be conciously 'steered' to run in a straight line - at which point the bearings are overly-tight; back off the adjuster a skootch.
The other way to organize this test would be to tighten the head bearings first, (if they are properly lubed, and not indexed or notchy-feeling) and feel the slight increase in drag as the forks swing with the front wheel clear of the ground. The bike will then more likely pass the above test, because you have already addressed the likely cause of feeling loose and prone to shake it's head.
