There doesn't seem to be an English word describing the ability to tell truth from a lie. I've Googled, and posted on my FB account, but can't seem to find a word that precisely describes this ability.

• Discernment. M-W defines this as "able to see and understand people things, or situations clearly and intelligently." Best so far, but a bit off the mark. It does not precisely identify the ability to perceive and separate truth from falsehoods.
• The antonym of gullible
• A discerning ear
• (Slang) anti-bs meter; able to discern bullsh!t.

Others have suggested guarded, prudent, realistic, astute, and perceptive... but none of these precisely describe the quality that I've described.

The word should fit 'XXXXXXX' in the following:

John is so gullible; he'll believe anything. Not so Jane, she is XXXXXXX. She has quite the discerning ear; you'll never be able to deceive her with a lie.

Here are some possibles (consulted thesaurus.com; listed antonyms for gullible; definitions from Google.)

• discerning. Having or showing good judgment. Discussed above; best so far, but a bit off the mark.
• astute. Having or showing an ability to accurately assess situations or people and turn this to one's advantage. Rejected because does not account for lies.
• knowledgeable. intelligent and well informed. Does not speak to distinguishing truth from lies.
• perceptive. Having or showing sensitive insight. Does not speak to discernment of truth or lies.
• suspicious. Having or showing a cautious distrust of someone or something.. Speaks more to distrust than separation of truth from lies.
• unbelieving. Not believing someone or something. Does not speak to the ability to distinguish truth from lies.