Possible Duplicate:
How do I detect that an SDK app is running on a jailbroken phone?
I found out that PayPal's Mobile Payment Library is not supported in jailbroken iPhone - see https://www.x.com/search/site/Mobile%20Payment%20Library%20is%20not%20supported%20in%20jailbroken%20iPhone and Paypal error "DEVELOPER ERROR: FILE SYSTEM CHECK FAILED" in iphone device
What I wonder is what API they use to know that the phone is jailbroken?