Finding vent hole in wing skin

We have a Lak 17 in the shop and have found that water cannot
be put into the left wing. It appears that during some previous
repair, apparently the shop glassed and refinished over the vent
hole in the underside of the wing. The problem now is how to
find the exact spot to drill in order to open up that vent hole.
Hopefully the vent line is still attached to the inside of the wing
skin where the hole was.

Measuring the position of the vent hole in the other wing is no
guarantee, and even measuring the location on the same wing on
another Lak 17 would be questionable. Tapping on the skin
seems to give no real indication either. So has anyone ever ran
across this before and if so did you come up with a fool proof
method of finding the exact spot to drill?

M Eiler