What do you do when you forget a word in IELTS Speaking?

What should we do if we forget one word in the Speaking test but still know what we want to say? For example, if you can't remeber 'thermometer', can you just say 'the thing used to check temperature' and carry on? Or should you stop and try to find the exact word? Not sure how much detail is enough for the examiner to understand. Also how do you practise this at home? Would describing an object without saying its name help? If anyone has a small excercise for this please share.