Rashi on Genesis
האמת אתכם means WHETHER THERE BE TRUTH IN YOU — therefore the ה has the vowel Patach, because it is a kind of question. And if you do not (ואם לא) bring) him חי פרעה כי מרגלים אתם BY THE LIFE OF PHARAOH, SURELY YOU ARE SPIES.
Or HaChaim on Genesis
שלחו מכם אחד, "Despatch one of you, etc." They did not do this as they were concerned that their father would worry needlessly about the fate of all the remaining brothers.
Radak on Genesis
if you indeed speak the truth, as you claimed when you said that you are כנים, truthful, above board.