How to check a dishwasher's drain valve and its solenoid:

Typical dishwasher drain valve solenoid Testing a dishwasher drain valve and solenoid
Disconnect the power source to your dishwasher before you conduct this or any other check. Either unplug the unit from the wall outlet, remove the appropriate fuse from the fuse box, or flip the appropriate breaker in the circuit breaker panel.

Your dishwasher will either have a single direction motor or a reversing motor. ONLY dishwashers with a single direction motor will have a drain valve. You can tell which motor you have simply by looking at it. If your motor has two or three wires, then it is a single direction motor. A motor with four wires is a reversing motor. If you have a reversing motor, you can skip this section because it will not apply to your dishwasher.

You can gain access to your dishwahser's drain valve by removing the lower kickplate/access panel. This panel is usually held in place with just a few screws. Remove the screws and you can remove the panel. Depending on your model, you may have to open the dishwasher door in order to remove the screws, but then close it again to remove the panel.

Typically the drain valve is located in close proximity to the unit's motor. It can be found elsewhere, though most times it is found close to the motor. There should be a hose running from the motor that leads to the drain valve. You will find that your drain valve consists of a gate arm mechanism and solenoid. Sometimes the solenoid is referred to as a coil. The gate arm should move up and down easily. Try moving it now. Notice whether or not the movement is smooth. Also, notice the condition of the two springs attached to the gate arm. If either spring is missing or damaged, replace it.

The solenoid is the component that engages the gate arm. It is connected by two wires. Label each wire before disconnecting them. When you go to disconnect the wires, you'll notice that they are connected with slip-on connectors. Grasp these connectors and pull firmly. Do not pull on the wires themselves. It might even be a good idea to use a pair of needle nose pliers to help you.

Use your multimeter to test the solenoid for resistance. Set your meter to the R x 1 setting. Place each of the meter's probes on one terminal each. When the probes touch the terminals, the reading should change from a reading of infinity to a reading of approximately forty ohms. If you receive a reading of infinity, or a reading that is no where near 40 ohms, then you will need to replace the solenoid.