Tachometer Pos or Neg?

["Daryl May" <mayfam@sprynet.com>]

My tachometer comes off of a 1967 Midget donor car, which has long since
been stripped of electricals and related labels.

I am wondering whether the tacho is pos or neg ground, since I gather that
1967 was a changeover year.  Is it possible to tell by looking at anything?
 Or, to get to the point, how can I make it negative ground no matter what
it is now?

Thanks,

Daryl