Step 1 · Observe and Confirm Symptoms

The first step is to confirm the exact nature of the problem. Go into the bathroom and turn the shower on. Let it run for a minute or two. Observe the toilet bowl water closely. Listen for the distinct 'gurgling' or 'glugging' sound and watch for bubbles or the water level dropping and rising. This confirms that the draining shower is creating a vacuum in the shared drain line that is pulling air through the toilet's P-trap.