You would need the source code including the libraries, assets and resources used in order to be able to pass it on to somebody. Here are some tips to not allow somebody to cheat you.
1) Do not let them know that you are not knowledgeable about source code! In fact, say that your technical team will review each and every deliverable.
2) Make a clear agreement with payment terms linked to delivery of source code to you.
3) If possible, try to get somebody technical to have a random check of the deliverable just to make sure that the right things are being delivered.
4) Demand a daily or weekly delivery to not allow the development team to operate as a blackbox.
5) With-hold a good fraction of the payment till a properly working app along with full source code is delivered.
6) It might be good to have some kind of a shared source code repository running CVS or SVN that would allow them as well as you to access the source code any time.
7) In the long term, employ somebody who can review technical stuff for quality and completeness.
Hope this helps.