The most common sign is a toilet that stops up occasionally that can be cleared using a plunger. This is not a very definite sign of a root problem but it can be the first symptom.
With all of the low flush toilets on the market a lot of people have a stoppage here and there. It doesn’t always mean there is a problem with the drain line, but it is an indication that you should keep an eye on things.
If the toilet stoppages become more frequent and more stubborn, the chances that you have a roots in the drain problem becomes more likely. In some cases the toilet will eventually become so blocked that it cannot be cleared by using a plunger. Again, the occasional stubborn stoppage is not by itself an indication of a problem. What you need to look out for is a recurring or worsening problem.
A localized drain problem is another sign of roots growing into the drain. It is easy to check whether the problem is localized to one particular toilet. If your toilet is stopped up the first thing to do is to make sure the other fixtures or other toilets are draining.
If other plumbing fixtures are also stopped up or the drain waste is coming up into your showers and tubs then you have a main line stoppage.
A main line stoppage needs to be cleared through the clean-out, if you have one. If the stoppage is affecting only one toilet you should consider investigating whether the cause is roots in the drain line. http://www.plumbingsupply.com