A variation on a couple posts above; never lend a knife (or anything, especially money) if you would not give it to them.
If you get it (or whatever) back in bad
,good or even better
condition (e.g. a full tank of gas-car, not knife) it will tell you much about their character. If you don't get it back, well, you would have given it to them anyway and now you know. 



