| As far as I know (and Nelly shown me) there isn't many ways to see but to take the cams out, then check for any worn out chrome on the rockers. Any tiniest wear is a sign that it needs changing.
A good indication is when you take the covers off to do the clearances. If the cams show little wear being little shiny in the centre but still with their black colour on the outside, this is a good indication that the friction is OK with the rocker. When the rocker wears out, the cam starts wearing out too being shiny all the way.
However this method would only highlight a rocker that has gone already... If it is just on its way and hasn't marked the cam yet, you may be in for a surprise before the next time you open to check (broken rocker = BIG OUCH!! or worn out cams = ain't cheap either!).
Relying on valve clearance to assess dammage could be inacurrate cos it might be the result of of other things. I had quite a build up of monoxide on the valve for instance. | 
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