I already looked into all the places in this area.
I already did my homework on this area.
この場合、次のような言い方ができますよ。
ーI already looked into all the places in this area.
「この地域はもう全部調べたよ」
ーI already did my homework on this area.
「この地域はもう下調べしたよ」
to do one's homework「下調べをする」という言い方もありますよ。
ご参考まで!