In both the front and rear tubes, the heater voltage is on pins 7 and 8. If you look at Elise's sockets from the front and start from the 'notch' that helps to insert the tubes correctly, they are the first and second holes in the socket to the left. Wait: here's a picture showing it clearer - it's the socket holes with the red lines next to them. It's the same on every socket. BUT: the voltage must be measured with tubes in. So one of the tubes must be inserted 'shallow' to enable contact with the measuring pins of a voltage meter. I
wouldn't want any of you to experiment if you are not comfortable to 'basic electronics' - there is a risk for a shortcircuit if you touch two pins with one measuring pin of the voltmeter.
WAIT: there's an easier and more secure way for those of you who have EL3N tubes in use. The adapter for those has readily accessible copper contact points on top, directly next to the contact points of the tubes. They are accessible during play with tubes inserted normally.
This is the pinout for EL3N tubes:
Again the two heater contacts are indicated with the red lines. EL3N tubes have two 'sets' of four contacts, one set closely spaced, the other wide. You need to measure between the two center pins of the narrow spaced set.