There is a tracking thread in Webkinz Insider:
http://www.webkinzinsider.com/forum/f166/lets-track-dog-official-doug-hunt-511034/Make sure you click on last at the top so you can find out the latest info.
Secondly, there is a pattern to this.
1. First time you see Doug and you have his gem, you get rocky road ice cream
2. Second time, Rocky candy
3. Third...Gem belt
4. Fourth...Helmet
5. Fifth...I think it's the overalls, but I'm not completely sure.
So, another thing I've found is even if you don't have his gem, sometimes certain accounts have a glitch where it awards you the prize anyway if you click yes. I have one account that does this.
Another tip is to switch pets, and go in and try again. You can do this up to I think 3 pets before you have to log out and log back in your account to try again. I wouldn't recommend doing more than 3 pets unless one of them didn't have the gem and you still want to try. It keeps track of each pet and what place they are in that list. If you hadn't used a pet before, you start at rocky road ice cream.
Hope this helps!