Greater Yellowlegs are a fun bird to work on ID skills with since they are difficult to differentiate from Lesser Yellowlegs. However over time, this is one of those birds that can be identified by "Jizz". I know, that sounds gross but it is not really. Jizz is the birder's word for Gestalt. If you don't know that word either then look it up!
Longer slightly upturned bill is a good field mark.