Take a guess! It's something you've been wanting.

Take a guess!
当ててください！

Take a guess! It's something you've been wanting.
当ててごらん！あなたがずっと欲しがってたものよ。 

take a guess on someone's age
（人）の年齢を当てる

take a guess on the number of
～の数を当てる

Let me take a guess.
（それが何か）当ててみるね。


"There's a guy here who's got the first prize." "Wait. Let me take a guess."
「この中に一等賞を取った人がいます」「待って、誰だか当ててみるから」

take a blind guess
当て推量をする