I keep on telling them "Make it spicy" and then I wonder if they actually just gave me the mild version :<
I've found two methods that often (but not always) work to get a restaurant where I am not already known to provide legitimately spicy food (assuming they are capable of producing such).
The first is to tell the waitress up front that if they manage to make it too spicy you'll not only pay for the too spicy food, but also pay for a 2nd order of the same with a more appropriate spice level. This often avoids the 'play it cautious so they don't have to re-make the food' issue.
The 2nd is to ask the waitress to tell the chef that you don't believe he can make it spicy enough for you. Many chefs will go out of their way to attempt to make it too spicy in the face of such a challenge, and will often come out to watch you try their food to see if they succeeded.
And please don't try either of these 2 methods if you believe that the restaurant might actually be able to deliver food which is too spicy for you to enjoy. I've never yet found a restaurant that can in my case, but I'm one of those people who wins bar bets in Thailand by being able to eat spicier food than the locals and thinks grilled ghost peppers with a bleu cheese dipping sauce make a great snack.