With the basic search command (with regular expression checked), I could able to find the instances where there is period followed by any word or number by the expression \.\w And with the expression \.\u I could able to find period followed by an upper case letter (preferred) without space in between.
But in the replace window if I give \.\s\u it doesn't work. It replaces the capital letter with the letter 'u'.
I suppose if I write a extendscript it may work. Or
Is there a way to add the expression \.\u for the auto spell check to find, thus a red squiggle will appear on these instances?
You can also tell Fm to flag two words with a period but no space in between by disabling Words Containing . in Edit > Preferences > Spelling Options. When you right click over the squiggle, Fm will offer the correction with the space.
But I'd use the RegEx that Rick provided as part of the document clean-up if you have a typist who repeatedly forgets to add spaces between sentences.