Notice: winglets (flicks at the end of wings) are too unreliable to use because they can change with variants. Please ignore any comments to do with winglets! However, Airbus tends to use y-shaped winglets and Boeing tends to use tall winglets.
You can only immediately identify some obvious aircraft, like the A380 or the 747 with two floors. This document goes over the distinct differences between aircraft and how to use process of elimination to identify aircraft.
TL;DR decide if its Airbus or Boeing, decide if it's short/med/long and guess!