Here's a Corvette tip that's applicable (they've had aluminum rads since 1963). With engine off, remove the rad cap and stick a VOM probe in the cold coolant with the other probe grounded. If your meter reads 50mA or more of current flow from electrolysis, use some type of anti-acid in your coolant (rad shops recommend No-Rosion), or add a sacrificial piece of zinc or magnesium.
Note you must use a good VOM- a $6.95 Harbor Freight assembly is not accurate enough. Some local water is so acidic it will dissolve a Coke can from the outside!