Same pattern elsewhere
Anyone/any one, everyone/every one and maybe/may be follow the same logic: the one-word form is a single part of speech, the two-word form is a determiner plus a noun or verb. “Maybe I’ll go” versus “It may be late”.
Catch it automatically
The grammar checker flags common mix-ups like this one by looking at the words around them, and the sentence checker can explain any sentence you’re unsure about.