The most unpleasant symptom of all – foul smells are a dead giveaway for a blockage. If something has blocked the drain and begun to rot, you will certainly know about it. The bad smell may well begin to occur before any visual symptoms emerge, so if you start to catch whiffs of something being a little bit off then don't ignore it.
Finally, if your pipes and fixtures sound different to usual, it could be an indication of a blockage. Gurgling noises from drains, plug holes, toilet flushes and pipes are early warning signs – the sound is created when air trapped in the pipes by the blockage is displaced and pushed up through the system.
Any one of these symptoms on their own is cause for concern, but if you're noticing two – or even all three – then you may well have a plumbing emergency on your hands.