1. Two patterns that can help you know when to choose -ence rather than -ance:
a. Stems that can form adjectives ending in [enshəl] spelled <ential> will form nouns with -ence, as in confidential and confidence.
b. Bound stems that form nouns ending in [əns] usually take -ence, as in patience.
It would be easy if we could just say that everyplace else you should choose -ance. Alas, it is more complicated than that, though there are some things we can say that can directly help you know when to use -ance.
But nouns that end in either -ence or -ance very often have a partner word, an adjective that ends in either -ent or -ant. For instance, the noun confidence has the partner adjective confident. And if a noun ends in -ence and has such a partner adjective, the adjective will aways have -ent. If a noun ends in -ance and has such a partner adjective, the adjective will be have -ant.
This does not directly help us choose between -ance and -ence, but sometimes we can remember how to spell the adjective but not the noun, or vice versa, so it can help to remember that -ance goes with -ant while -ence goes with -ent.
2. Fill in the blanks. Some of the adjectives can also be used as nouns:
The OED tells the tangled tale of -ance and -ence in two tight paragraphs, at -ance and -ence.