When an attack roll is made against the creature, it can use its reaction to roll the Bardic Inspiration die and add the number rolled to its AC against that attack, after seeing the roll but before knowing whether it hits or misses.
In my time as DM I would always say, for example, "18 to hit?" unless it seemed absolutely clear to me that an attack hits. However, my current GM prefers to keep track of our AC, and simply tell us the result without calling the results of his attack rolls.
These are the possible solutions that occur to me:
- GM must consistently call '## to hit?' waiting for the player to respond 'Hits!' (Positive: Most useful for the inspired character. Negative: some immersion lost.) This is my preferred approach.
- GM must remember who has inspiration and at least announce his attack-rolls against them (Positive: best of both worlds. Negative: GM is likely to forget)
- GM does nothing; player must gamble on the use of their inspiration based on their own guess, intuition, or reading of the DM, and announce this immediately after the GM rolls attack. (Positive: Best for immersion, easiest for GM. Negative: the class feature becomes much less powerful. All of the onus is on the player to remember and interrupt the GM). I believe RAW rules out this solution, which is the one preferred by my GM.