If I understand the question correctly, i have a digital torque wrench that tells you the torque value you're at while you apply force. I suppose you could use something like this and try and carefully note at what reading the wrench starts to move the existing bolt tighter?

That said, I wouldn't bother. For seat rails, just start with something average like 6Nm, maybe using some assembly compound. If it doesn't slip, you're good.