Well, there were a select few on here that I thought were worth trying, very, very select few. But when I need a Truth spell(and this is not on the site, I checked) I used a fire one.
You will need:
Flammable alcohol or liquor
Matches or lighter
A shot glass
A picture of the person who you want to see is lying(or not.)
The Picture needs to be cut small enough that it can fit in the glass, but try to keep as much of the picture in tact as possible(mainly the face.)
Fill the shot glass almost to the brim with the alcoholic/flammable liquid, then place the picture to float on top of the liquid.
With you finger, wet the brim with the flammable liquid, quickly dry your hand off, and light the brim with your match or lighter(you may have to repeat this a few times to get it to light, because the liquid will evaporate quickly, but you also don't want to light your hand on fire).
-If the picture catches fire, they are lying.
-If it does not, then they are telling the truth.
-If the picture sinks(after the fire is lit, not before), then they haven't lied, but they aren't telling you the truth either, keeping some sort of secret.
Hope that helps.