assuming it's a conventional wind vane something like the old Met Office
'chicken on a stick' emblem, then on a sunny day at mid-day GMT
(1300BST) note the direction of the shadow of the wind vane centre pole
and align the shadow of the N-S bar with that shadow, putting S on the
sunny side.


Unfortunately, the Sun is not necessarily due South at 12:00GMT (13:00BST).
The Sun varies in Azimuth throughout the year due to the Earth's eliptical
orbit.

As Tom says, an accurate compass or the Pole Star is the best way to find
North.

Alternatively, use the TV-aerial aligning technique.....
Just point your weather-vane in the same direction as everybody else's!


Brendan