Max out your MAC
It’s best before sending any private information with an OTP to send a Challenge-Response message first. Technically, you could agree on two code words to use with your recipient: one could indicate that it’s you and you’re able to talk freely — for example, “Everest” — and another could disclose that you’re talking under duress — for instance, “Sparrow”. This means, however, that the same text is being encrypted each time, making messages easier to crack.
A better system is to choose from a list of prearranged words or phrases. One way to do this would be to agree on a book — directories and almanacs are perfect types of book for this — and in your ‘challenge’ message, you could send an arbitrary page and line. For example you could agree to use the 1992 edition of Wisden Cricketer’s Almanack. Bob can message Alice saying, “613-1.” Alice can reply with the first line of page 613, which is:
“Worcestershire were the only county to win two trophies in 1991.”
She can then add a challenge of her own to the message, for example asking for page 582, line 4.
If, of course, Alice replies with anything other than the correct words, Bob will know that it’s not her or she’s under duress, and the same applies with his reply to her. You can further increase the security of this system by agreeing beforehand that the response to the challenge should not be the line requested in the message but the one (say) three lines after, or perhaps the same line on the following page.