Vanagon and Eurovan instrument clusters have a warning indicator that comes on when the alternator has stopped charging. On all Vanagons and early-model Eurovans, it is a little round LED with a diagram of a battery next to it. On later-model Eurovans, the actual diagram of a battery lights up on the instrument display. This indicator comes on each time you turn the key to the ON position, and it goes out when the engine starts and the alternator begins to spin and make power.
If, while you're driving down the road, this indicator lights up, it means (at the very least) the alternator is no longer replenishing the battery. That means the power required to run the engine management system, fuel delivery system, and all other electrical systems is coming entirely from the battery. If it were just a matter of the alternator failing, this would not be anything to panic about. You could simply look for the next exit, get off the road, and take a look. However, this indicator can also mean BIG trouble is coming... and it's coming SOON.
If the alternator suddently stops charging, it could mean the V-belt that runs it has broken or come off for some reason, which would make the alternator stop spinning and trigger the light to come on. That, in and of itself, is not reason to panic, except for one important detail: The same belt than spins the alternator also spins the WATER PUMP, which is responsible for pumping life-saving coolant through your cooling system! Thus, if this light ever comes on while you are driving down the road, start watching the coolant gauge like a hawk while simultaneously looking for a safe place to pull over just in case. If that coolant gauge starts to climb: PULL OVER IMMEDIATELY. You have literally seconds to shut the engine down before the cooling system boils over—or, even worse, severe engine damage occurs.
If you have ever had this light come on, ignored it, and ended up having to buy a new engine as a result, there is little doubt you will ever look at a little battery diagram again without muttering a few curse words under your breath...