that is the most accurate correct method with the heads on / in the car
for a 'just to see if it's close' method you can use a drinking straw in the plug hole touching the piston & turn the crank through by hand while watching
if the balancer has spun it's probably more than the few degrees that you wouldn't be able to tell by the straw method
paint stripe across the rubber is a good idea
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.