A casual way to ask how someone is doing or what's going on (short for “どうしている?”). Common in conversation.
久しぶり！どうしてる？最近忙しかった？
Long time no see! How have you been? Have you been busy lately?
彼、どうしてるか電話してみるよ。
I'll call to see how he's doing.
どうしてるって聞かれても、特に変わったことはないよ。
Even if I'm asked 'how are you', nothing in particular has changed.