How do I know whether 毎月 should be まいげつ or まいつき

Question

This is my first question here and I'd like to ask something that has been bothering me for a while.

Some words written in kanji can be read using both onyomi and kunyomi of some of the kanji and the meaning of the word stays the same. An example can be "every month" - 毎月 which can be read both まいげつ and まいつき. If such a word is written only, how does one know how to pronounce it? For example when reading a written text aloud.

EDIT

It seems that the question is not very clear/too broad. Let me narrow it down and ask specifically about 毎月. Don't worry about any other words.

If I have a written sentence

この雑誌は毎月出る。

How do you know to read it:

この雑誌は毎月【まいげつ】出る。

or

この雑誌は毎月【まいつき】出る。

Answer 1

I'll quote part of Tokyo Nagoya's comment:

I say it まいつき 100% of the time and hear others say it the same 99.99% of the time.

So it seems that まいつき is the common reading. Other speakers corroborate this, with one saying that まいげつ is rare and another saying that まいげつ isn't even an acceptable reading—although I'm not willing to make that claim myself.

(However, if you ever have occasion to read 毎月抄, you should read it まいげつしょう.)

Unfortunately, there's no universal rule for deciding which reading to use for a compound when more than one reading is possible:

1. You want to pick the reading that makes the most sense in context. (In this case, both readings represent words with the same meaning.)
2. You want to pick the reading that the author intended, so if one reading is more likely (in general or in a particular context), you should favor that reading. In this case, the favored reading is まいつき.

Unfortunately, you'll have to memorize this on a case-by-case basis.