I think of words like bookkeeper or foghorn. Then there a number of words which are hyphenated. Some Basic Patterns First of all, right off the top I must say that English is not completely consistent. The best solution is to look up the word in a good dictionary. If there is more than one spelling listed, you almost always do best to choose the first one.
Bumblebee is listed first because it is the most widely used.
Having said this, there are patterns that we can begin to see. Most often, compound words, unless they are quite old in origin, remain two words. If the roots have Germanic or widely used folk origins, the compound word is much more likely to be a single word. It is also more likely to be combined if it can meaning something else when divided into two words. Long-Time Hyphenated Words English at its root is a Germanic language.