When can I use a cake recipe to make muffins?

I can suggest two and a half ways to check.

1. Muffins are closest to pound cake. The closer the base recipe to a pound cake (1:1:1:1 sugar:flour:eggs:butter by weight), the more likely that it will give you something muffin-like. If there is a liquid added, this can still work quite well.
2. By thickness. If a recipe is supposed to produce a thick and spongy cake sheet, it will likely make something that works as a muffin (some people might tell you you're making cupcakes then, but this type of discussion is rarely constructive). What won't work are thin or hard cakes - making prinzregententorte or dacquoise in muffin shape, if possible, is an undertaking for somebody with serious understanding of recipe design and a lot of time on their hands.
3. The half criterion - try to stick with chemically leavened cakes (baking powder). Cakes leavened by other methods, for a genoise, can turn out to be structurally difficult to work with when the shape is changed, and almost certainly won't give you a muffin top. You have more leeway here though, so if you feel adventurous, you can try recipes of this type and see what happens.