Yay! This topic again!
I'm going to try a new tack:
1) If making Insight checks as a kind of lie-detector works for you, go for it. And once you've decided to do this, you may as well let your players use "Can I roll Insight?" rather than circumlocute around it, trying to avoid using the metagame language. (Same goes for traps, by the way. If it works for you to "roll to detect traps" on every chest and dungeon door, go for it.)
2) What
I prefer, though, is that lie detection requires magic. People are no better at detecting lies than they are at picking stocks. I figure that anybody who assumes somebody else is lying is going to find some evidence to support it, and when they assume the other person is telling the truth, they are going to find evidence for that, too.
So instead of "using Insight" to detect a lie, let your players, if they are suspicious, come up with a plan for how they plan to
catch the NPC in a lie. For example (none of these are meant to apply to a specific situation):
- I'll hide across the street and see who comes in. (Stealth check?)
- I'll try to sneak a look at this ledger and see what the last entries are. (Slight-of-hand check.)
- I'll keep asking for more details about (insert context), and then keep coming back to previous details, to see if he changes any of them. (Investigation check?)
- He seems unconcerned about the murder; I'll chat him up and see if he reveals any concern about the victim. (Insight check?)
- I'll check his stables (Stealth?) and see what I can tell about what horses have come and gone (Animal Handling?)
Etc.