A number of the hardest to track-down problems I've come across with my iPhone application have only exposed themselves on jailbroken handsets. Is there a way to detect these handsets looking only at the crash logs?
This is kind of like this question but after the event rather than during...
The only thing I can think of is mobilesubstrate prints to the console a lot and 99.99% of jailbroken phones should have it installed by default and it usually notifies you when it hooks a phones function
While it certainly isn't a foolproof solution, you can look at the running binaries to see if there are jail-broken apps running at the time of the crash. Winterboard, for example, is an app that only exists on jail-broken devices. If you suspect a crash is happening on a jail-broken device, I would start by inspecting there.
you could also look for 3rd party app installers as well. Most jail-broken devices have those to load apps more easily. I would create a small list of apps that might trigger an 'aha!' moment, that way you will at least be positive it is jail-broken