If detecting falsehood and discovering truth are not the same skill in practice, then practicing honesty probably makes you better at discovering truth and worse at detecting falsehood. If I thought I was going to have to detect falsehoods - if that, not discovering a certain truth, were my one purpose in life - then I'd probably apprentice myself out to a con man.

I've heard from many sources that con men are actually among the easiest to deceive. The rationale seems to be along the lines that con men have utter contempt for their marks, and therefore once a con man thinks of you as a mark he'll be oblivious to any signs that you're actually the one playing him.

(I've also heard, separately, a saying to the effect of "It is difficult/impossible to deceive an honest man", but it comes with no justification other than perhaps a religious one.)