Clogged drains are no picnic, but the severity depends on the type of clog. Local clogs affect only one drain, while main clogs can affect your entire home.
Signs of a Local Clog
A local clog usually won't have much effect outside of the specific drain affected. You shouldn't experience multiple clogs at the same time. If you do, they will tend to be in a shared "zone." For example, the bathtub drain and sink drain will both back up or drain slowly, but the kitchen drains on the other side of the home will work perfectly fine.
Local clogs often come with some sort of warning, as well. The drain will begin emptying more slowly at first, or you may have gurgling issues from the drain. Sewer odors aren't usually an issue, either, although there may be foul odors in kitchen drains if food is the blockage culprit.
Signs of Main Clog
A main clog typically affects multiple drains, although they may not be affected equally. Often, those drains closest to the main clog will suffer full backups first, while those further away may just drain more slowly. All the drains in the home will usually show some signs of problems, though.
Sewer odors are also more prevalent with a main clog compared to a local clog since the clog is preventing raw sewage from exiting the house. Gurgling from various drains is also a common issue with main clogs as water and waste try to work past the clog in the larger pipe. You may also notice wet spots in the yard since main clogs are often accompanied by leaks. Basement drains or drains in the lower levels of the home may also back up, even when that specific drain isn't in use.
The symptoms above can help diagnose whether you are dealing with a local or main clog, but they aren't foolproof. For local clogs, your plumber can use non-invasive techniques at first to clear the clog, such as an auger. If these don't work, then chances are the clog is further down the pipe and may be in the main drain line.
One of the most accurate diagnostic methods is a sewer pipe camera. These can be used to determine not just if the main drain is clogged, but the exact location and cause of the clog. This way your drain cleaning service can specifically target the problem and make any necessary repairs so it doesn't happen again.
Contact a drain cleaning service if you are having issues with your home's drains.