For me, the simplest way initially is to remove the rocker inspection covers and the spark plugs. Select 4th gear and turn the motor over slowly using the rear wheel. Check to see all four valves are rocking, then increase the speed a bit to see if that makes any difference or you can hear a noise. Check also to see if the tappets are not bashing the covers. Also, a bright light down the plug hole will show whether the valves are opening or not (you can just see them). If all of this is ok, then it's time to look elsewhere for the issue.
For reassembly of the push rods, I used Colsbeeza's 'comb clamped to cylinder head' approach. I use elastic bands to keep the rockers AWAY from the push rods with the adjustment completely backed off. When the head is nearly on (just enough to remove the comb and with pushrods correctly located in the comb), I gently remove the bands and move the rockers toward their adjustment position - then set gaps accordingly once all is in place and checked as above. The logic is tha if the comb is secure, and the pushrods are in the correct places, then the rockers should just drop down on top of them. Tried this a few times now and it seems to work very well.