Why don’t you put the correct answers on the whiteboard while your students are still busy with the task? Here are some of my favourite takes on doing it without revealing what the correct answers are too quickly (my students are a clever bunch, always looking for clues, so it’s not easy to outsmart them…).
If the answers are numbers or letters, put them on the whiteboard in a circle, starting from the third or fourth answer – not from the beginning, as if your students are anything like mine, they’ll be watching you like hawks.
Once they are done with the task, add an arrow around the circle, going clockwise and starting from the first answer. Your answer key is ready.
If the task your students are currently busy with is a gap-fill task, you can put the correct answers on the whiteboard, mixing them with words with similar spelling but different meanings (than/then), words with the same stem but different parts of speech (adjective/adverb), pronouns (her/hers, them/their), easily confused words (adapt/adopt), verbs (has/had, is/are), and other distractors.
Once you’re done, your students have the opportunity to correct themselves or at least mark the answers they came up with that are not on the whiteboard so they can ask if they might be alternative answers or are simply incorrect.
Then it’s time to check the answers one by one. Elicit the answers, let them help you clarify why each answer is correct and others are impossible in this context and while you’re doing that keep marking the correct answers. Your answer key is ready for students to check once again they’ve got all the correct answers now.