Yes, there is an easy way. First make sure the amp is in the mode that runs all the power tubes.
Now look very closely at the tubes when the amp is running - if you look through the little slots in the plate structure, you should see some blue glow. Turn the standby switch on and off and you should see it come and go, if you're not sure.
If there is any tube that doesn't show the blue glow, it may be a dead tube or a blown screen resistor. Swap the dead tube with one of the good ones and try again - if the same socket still shows no blue glow, chances are the screen resistor is blown.
It's possible that a resistor could be overheated and out of spec, but still not quite blown - but that won't normally make much if any significant difference to the output volume of the amp, it would have to be so far out of spec that it would probably just fail anyway for that to happen.
Or...
If you want to check with the multimeter, the screens are pin 4. If you pull all the tubes and measure from pin 4 of one socket to pin 4 of another, you should get twice the screen resistor value - all the resistors are tied to a common supply point at the other end. If you get one socket that doesn't have a reading to any of the other three, it's got a dead resistor. If you're going to do this *MAKE SURE* the filter caps are discharged first - the screen supply is directly connected to one of the highest voltage points in the amp.
