No, good point.
OK. On a basic level you won't need to. Remember how I mentioned (or possibly you don't) that your skills are how good you are under pressure? So long as you have fifteen per cent in a skill, it automatically succeeds in low pressure environments. So if the guy in the shop tells you he gave you enough change or a prostitute calls you "handsome" then you'll just be able to tell. I'll mention it to you.
In a pressure situation its a bit different, though. As I read it, its a passive skill. Its not that you're scrutinising them consciously, just that you're good at picking up the little tells. So the way it would work is actually based on their roll, not yours. Normally to tell a convincing lie, someone would need to roll either under their soul stat or under their "lie" skill. To tell a convincing lie to you though, they need to roll under their lie skill/soul stat but over your detect lie skill. So the same roll needs to be a success for them and a failure for you. If your detect lie skill is higher than their soul stat then obviously you'll automatically be able to tell - because they can't possibly make that roll.
So lets put an example there. Lying Bob has a "Lie" skill of 30 and a Soul Stat of 60. When you ask him where the nearest hospital is, he points down the street in the wrong direction.
If he needed to roll under his Soul Stat for this roll, normally a 1-60 would do it. However, to lie to you he needs to also roll over your "Detect Lie" of 50, so only a 51-60 would die it.
If this were a really important lie for Bob then he might need to roll under his "Lie" skill. Here you'd be able to tell automatically - in order for it to work Bob would have to roll under his skill of 30 but over your skill of 50, which is obviously impossible.
Does that make any sense? And does that fit in with how you'd imagined the skill - as a passive thing that requires no particular action from you. If you'd viewed it as more active, that you narrow your eyes and weigh up whether you believe them or not, then let me know and I'll rework it.