How to know you need to change hot water heater anode

Inspecting them is the only way I know but this may explain it.

Signs That I Need to Change the Anode Rod | eHow

Got smelly water or something?

This is city water. Filling the bathtub I see that the hot water is green.

I filled the bathtub with hot water and it is green. Does not smell. We have city water.

I have asked that this question be moved into the plumbing forum. What you are describing is not an electrical failure. Don

How "green " are we talking about? Fill the tub with cold water. Still green. Anode rod should have nothing to to with color.

Attach hose to drain valve on water heater. Open drain valve and let it run until clear. Tell us what comes out.

That anode rod has nothing to do with your issue. The anode rod prevent corrosion from eating the tanks insides. I think you have a algea problem. Fill a pitcher with your garden spigo to see if there is the same color.

Cold water is clear, not green. I will drain the water heater and report. However, is it possible that some of the copper fittings in the whirlpool tub are making the water green? It has been 6 months since I used the tub.

Do not drain the water heater, flush it.

The difference is, to drain you close the cold water input valve, open a hot water faucet to let air in and open the drain valve.
To flush, leave the cold water input valve open and open the drain valve. That way you expel the water in the tank under force and hopefully flush out any contaminants and sediment.

The advice to flush your heater is valid. Let me show you how. For long life and fewer troubles you should keep your heater clear of mineral build-up by FLUSHING NOT DRAINING on a regular schedule.
Attach a hose to the boiler drain at the bottom of the tank. With the pressure on, open the boiler drain and let it run until the water runs clear. You will see a spurt of red,(rust) followed by white or yellow grains,(lime or calcium carbonate). This shouldn't take more then a few minutes.
Do this monthly to keep it clear. Now flush out your hot water lines on ALL fixtures that are affected . Now pull each aerator and clean the screens. Be sure you put them back together the same way you took them out. Don't forget to flush it out every month. Your heater will thank you for it. Hope this helps and thank you for rating my answer,
Tom

When you drain a whirlpool not every drop of water is expelled. I think you had algae growing in the little that remained

I would try filling the tub with hot water and add a quart of liquid bleach. Run it for a few minutes and then drain. I am thinking this will cure future issues from coming back.