Take a long solid screwdriver (not the ratchet sort!), whilst the car is ticking over, place the 'screw' end of the screwdriver on the rocker cover and directly over a tappet. Then making a fist over the handle and placing your ear on the fist you should hear the single tappet; tapping! Now run the screwdriver across the rocker-cover listening to each tappet individually.
If there is a slight difference in tapping compared to the others, then it should be adjusted accordingly. If they all sound the same they should be alright!
Give it a try...!
