When I was there with binoculars I was using the patterning on the top of the head to tell them apart but you can't really see that in the picture. It seems the length of the bill is a far simpler way to tell them apart though.
yep, the bill of the curlew is incredibly long - once you're aware of the difference there's no confusing them with whimbrels even from a silhouette. Curlews of course are much bigger birds as well, but sometimes that can be hard to judge in the field if you're not used to them.
