I wanted to monitor the charge in the 4D cells powering my D7 (and later my SBM-1), so I marched right into Walmart and bought a cheap, digital multimeter. (You can get one of these at almost any store like this. Radio Shack has them, but they'll be more expensive.) Mine was a "Popular Mechanics" brand and cost me about $11. It is about 3x4.5" and rests right on top of the 4D pack.
Next, I cut the ends off of the probes that came with the multimeter to expose the bare wires. I then soldered those bare leads to their corresponding polarity leads from the 4D cell pack (the same leads that supply the D7 and SBM-1).
There you go. Flip on the multimeter to the correct DC monitoring mode and you'll have up-to-the-second coverage of the remaining life in the batteries. (Some multimeters will be auto-ranging, others will give you the choice as to what decimal level you see the remaining juice. The cheaper meters probably won't be auto-ranging. Mine isn't and I prefer it that way.)
TIP1: Get an extra set of probes to carry around to check extra D cells or other batteries in your arsenal.
TIP2: Pay attention to the drain-times and you'll get more use out of your batteries. After a while I knew if I could get another show out of a set by checking the voltage and remembering how long my eq ran to get it to that voltage last time. It is still guess-work, but with a much greater degree of acuracy AND it reduces your stress over worrying about running out of power.
Yeah! You save money and fill up the landfills slower! (The next step is switching to a re-chargable unit of some-kind. See the first item in my list for producers of such products.)