Go to Preferences, and under advanced type is there an option for "Show Alternates"
I don't think that would do it. The Show Alternates feature only shows alternate forms like Small Caps, or Ordinals, or Swashes.
The poster wants to see the glyph alternates combined with different diacritical marks. You need to open the Glyphs panel (Type > Glyphs) and view the various Latin glyphs:
Yes, I can find the characters I need using Type > Glyphs but it is a cumbersome way.
It's funny. When I depress and hold the "o" key for a second in a Text window in InDesign I get a line of "o"s. But when I do the same thing in the Find/Replace window of InDesign, the unicode character menu pops up just like it's supposed to. Why would this feature work in InDesign Find/Replace but not in InDesign Text?
There's many things in InDesign and other apps that I ponder the same thing about.
Thing to take away from this is that it doesn't work as expected and you have to use a workaround for now.
You log a feature request/bug here Feature Request/Bug Report Form