holding tanks for heads

If the tank tops are accessible I would remove the feed hose and use the old dip stick method. For future use I would look into the sonic tank tank sensors. There are a lot of them on the market and they are easy to install if you have access to one side of the tank. Most of these are reasonable in price and will certainly give you a little piece of mind.

Eeuuw ... that's just plain nasty!

Just have them pumped out and flushed, they probably need it anyway.

Here's an overview:

http://www.sealandservices.com/PracticalSailorTankMonitorComparison.pdf

Eddie, if you have a hand pump toilet like Raritan or Jabsco, when it is full you won't be able to to pump the handle anymore. Don't worry about overflowing; they are equipped with check valves. You should be able to get a sense, before long, from when you pump out the tank to when it should start to approach the full level, and then just use a pump out station [or whatever there is in your area]. Of course if you're planning a long trip and you know it's been a long time since last pumpout, plan ahead and have it done ahead of time.

If the dipstick method is too objectionable, but you are able to gain access to a sounding tube or similar, you can shine a torch and use mark 1 eyeball to see the reflection off the surface of the liquid and how far down or up it is. You don't need to be taking big snorts of the smell and it doesn't take long, if needs be you can hold your breath long enough. The laundry/grey water tank where I work smells far worse than the sewage retention tank IMHO.

A lot of the fluid in the tank will be seawater from the flush.

Perhaps when it is emptied of effluent you could fill it with seawater and get an idea of how big it is and a ball park of how many flushes it takes to get to a level where you think it's high enough to get pumped.

Make it part of your routine that you pump it overboard every time that you're more than 12 miles off shore. Failing that make it part of your passage plan that you are so that you can.

Hard to believe there is not an indicator; often inside a cabinette in the head. Often it will have just a red & green indicator light (sometimes a yellow/orange to warn that you're getting close). Easy enough to install though if not. The most likely problem will be that the filter will clog and then the toilet will back up. Unlikely it will explode except when you open the cap to pump out (be carefull there. YUK!). Pump outs are generally not expensive (maybe $15) and often free when you fill up. Your tank should be pumped out every few weeks if used at all to hold down odors and to avoid waste solidifying in the tank and becomming hard to pump. After you pump out send in several gallons of fresh water and then pump that out.

I can't believe so many words are spend on this topic.
Please save yourself the pain and spend the US$ 200,- or so to get yourself a new sender and gauge.
I have worked on nearly 200 black water systems over the past 5 years and know what it's like to.

What you might want to consider though is to open the man hole in the tank and stir the sediments from the bottom with a paint-'mixer' on top of a battery drill. If all the sediment is suspended you can flush the tank. Often there is a lot of sediment laying on the bottom and so reducing the actual capacity of the tank.

+1 on installing the level indicator. Check the head to make sure that there isn't one already there. I can't believe they wouldn't install one from the factory.

Also, my holding tank is a white poly tank and is visable after lifting a hatch in the gangway. You can also see how full it is by the shading color difference of the tank.