scrolled window / viewport question

Ok so I've got this scrolled window. If I wanna stick a widget that
does its own scrolling, I can just do a gtk_container_add. If it
doesnt, I should do a gtk_scrolled_window_add_with_viewport (or
give it a viewport, but same difference). My question - is there
a way programmatically look at the widget and know which to do? The
widget going in is coming from a plugin, so I (in theory) dont know
which it is.

Slyder@xxxxxxxxxxxx | "Into the flood again...
Earendel / <insert here> | ...if I would, could you?"
Sing your dreams, Scream your song | --"Would", Alice in Chains