Collocations for doubtful - adj.

be, feel, look, seem, sound
Eric was far from sure and Marion looked doubtful.

become
remain

extremely, fairly, very, etc.
a little, slightly, etc.

about
She was rather doubtful about the wisdom of eating the seafood.

be, look, seem
remain

extremely, fairly, very, etc.
highly
Even if we could go, which is highly doubtful, John wouldn't be able to come with us.

increasingly
a little, slightly, etc.

Doubtful is used with these nouns:
expression, legality, look, origin, validity, value