Right now my system is to average all of the Hit Dice for all of the monster encounters and use that as my experience level. For example, in my Extra Stout adventure, the sum of all 11 monsters's HD is 42, so I rounded the difficulty level to 4 (42/11~4).
I'm not fully satisfied with this method because it fails to account for the many dangerous factors other than monsters. Traps, items, and other environmental hazards can adjust relative difficulty. This system also doesn't account for the probability of encountering certain hazards. This last problem is more pronounced in more sandboxy-style of play.
I'm not concerned with balance, but I am very concerned about giving potential customers an accurate snapshot of the relative difficulty. So, until I come up with something better, this is the system I plan on using. You can expect to see this begin with my next adventure, Menagerie of the Ice Lord. It has a difficulty level of 3, by the way, and should be available for sale before the end of January!
Here are some of my illustrations for Menagerie: