adverb
Polite equivalent of どう. Used to ask about manner, state, or to make a suggestion or offer.
お加減はいかがですか。
How are you feeling?
コーヒーは
How about some coffee?
na-adjective
Used in the pattern いかがなものか to express doubt or disapproval about something. Often implies that something is not quite right or acceptable.
その計画はいかがなものかと思います。
I have my doubts about that plan.