There are many techniques that can be used to find a clog in the plumbing system of a house, but we will share some of the more common ones with you.

A Sewer Camera
The easiest and most effective way to find a clog is with a camera. Being able to see inside the pipes is very helpful. Most modern cameras are also equipped with a locator which allows a person to scan above ground with an electronic wand like a metal detector and precisely pinpoint exactly where the camera is, leaving little doubt as to where the clog is located. The major drawback to the camera is if the pipes are currently full of water it can be nearly impossible to see anything in the pipes, which is why this is usually done after clearing the blockage. Once the pipes are drained the camera can inspect the pipes for damage which may have caused or contributed to the clog in the first place. To clear the clog we might use a sewer auger or even better a water jet to clear it completely- learn more about clearing pipe here Drain cleaning solutions

Flushable transmitters/Robots
You can use a flushable transmitter which is a device which emits a signal that can be found with a wand or other locator. You stick the device in the plumbing system, and it makes its way down the pipes until it comes to rest on the clog in the pipes. The downside is you cannot leave the device in the pipe you will have to retrieve it, or it may be the cause of the next clog in the pipe. Also, if the pipes are not draining at all it may be difficult to get the device to “flush” down the pipe. It would be easier and accomplish the same thing to blindly push a sewer camera equipped with a transmitter down the pipe until it stopped and then locate the camera and pull it back out. These devices may work better in locating a septic tank or other fixture that may be out of reach of a camera’s cable length, which is why it is not the go to solution for us.

Knowing the plumbing system of your house
If a homeowner knows exactly how his sewer pipes run and are connected underground and which way they flow it can make narrowing down the problem much easier. An experienced plumber can walk around and through your house and get a basic idea of how the house may have been piped which allows them to find clogs more quickly, as well. If you know where your house ties into the sewer or septic tank you know which way the pipes go. If the entire house is stopped up, then it is most likely something in the main line. If only 1 or 2 fixtures are stopped up, then the clog must be between where those fixtures tie together and where that pipe connects to the rest of the house. This is why when your toilet backs up checking to see if your sink or bathtub are backed up is also very helpful in finding your clog. Once you narrow down what part of the plumbing system is clogged you can then decide how to address it and where you will insert the drain cleaning tools.

Random drain cleaning
In the event you have a complicated or non-standard piping system in your home, finding the clog may be more difficult. Having multiple buildings or multiple sewer connections from a single house, having separate sewer systems under your house for different fixtures, or other types of unique situations can leave even experienced plumbers scratching their head. At that point running a sewer cable machine down each clogged fixture until you clear the clog, and everything drains may be the only way.

There are a few things that can make locating the plumbing clog more difficult.
Having more than 1 clog in the house
Not having any cleanouts on the house
Having a poorly piped or improperly piped house
Having extremely long runs of sewer pipe without any access points
If it is yard drains or storm drains that are clogged causing flooding the plumber needs to know where the drains are to find them under the flood water.

All of these things can be fixed, call a local plumber if you are struggling to find a clog.