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Ha, I thought this was a thread asking for whitelists when I read the title :3 I fail :3
It's possible by just hitting their steam profile link on their SG profile, and checking their wishlist there. Don't think there is any easier way.

EDIT: The thing TreeB linked seems pretty decent.

This might help.

Go on their steam Profile > Wishlist > Profit?

http://steamcommunity.com/profiles/<insert user profile id/number here>/wishlist

yes, you check sishlist on group. but you must make new steam group for your whitelist for temporary :D

There is a thread featuring a script m0l0 created which fetches your whitelist's wishlist:
https://www.steamgifts.com/discussion/TSa4B/whitelist-wishlist-and-tools-updated-may-11-2016

TreeB already linked it down there, but I thought I give it some more explanation, it can easily be overlooked.

There's also https://www.steamgifts.com/giveaways/wishlist
Doesn't give exact peoples wishlists, but can give a pretty good overview of what games are popular.