Symptoms of an imminently flat tyre include heavier than normal steering or a tendency to go straight on in corners. Sometimes it just needs some air, but more often than not, you’ll have a puncture.
Unless you ride an old Lambretta or Vespa scooter, you won’t be carrying a spare so you’ll either need to fix it temporarily or call the AA. Tubeless tyres (on most bikes except those with spoked wheels) are relatively simple to repair either with Tyreweld or a repair kit (sold at most bike shops) that glues a plug into the hole. The repair kit takes a little practice and sod’s law says that the first time you need it will be at the side of the M4 in a blizzard. A smart and forward thinking person would wait until their tyre needed replacing, ride to the bike shop which will be replacing the tyre and then put a nail in it so they could practice.
Faced with a flat tyre, the first thing is to find what caused the puncture and remove it. With that done, either squirt a can full of Tyreweld into the valve and get on your way (keeping the speed down to the recommended limit stated on the can) or ream out the hole, fit the repair kit plug and inflate with the CO2 canister supplied.