What is a general rule for predicting whether or not a bond will be ionic?
When considering if a bond will be ionic, I'd say look for two main things:
Note that if #2 is true, 1 will also usually be true.
Electronegativity, by definition, is the measure of the tendency of an atom to attract an electron to itself. Now, in a compound, if one element has a much, much higher tendency to attract an electron than the other, then the electron would spend more time at that atom. This would create more defined polar regions (i.e. + charged and - charged regions), which is what defines an ion.
Hope that helps :)