I'm not sure I have a test dish - I judge by whether the food looks, smells and tastes like I want to eat it. And preferably no silly plating or garnishes...
However, for desserts in a new place I'll go for the rum baba. It's very hard to screw up, but it's also very hard to make it memorable.
ETA: If I'm being honest though, I'll often judge a restaurant more on the service than the food. I've had a lot more good food ruined by lousy service than vice versa.