ORIGINAL: splatt
I used a small bubble level and placed it in the V and used a Robarts level on the wing. Get one of those 2-3 dollar bubble levels and use just the little capsule that's got the green stuff in it. Than you can eyeball the wing if you don't have a wing level. Try to get to as close to zero as possable.
I have a another solution that I think working very well. (see the pic) this is a tool that I made for gluing the two tail blades together. When mounting the stab on the fuselage
I level the plane so the wing is at o deg. then put the upper part of the tool that fits very well. Then I put a steel ball on it. It will shoe very fast if the stab is down or up wards.