First, you can retain a patent attorney in a nation where you suspect an application was filed, two other websites that are helpful are the European Patent Office (www.epo.org) and an more comprehensive international site by the World Intellectual Property Organization - WIPO (http://patentscope.wipo.int/search/en/structuredSearch.jsf)
I agree with the previous answer that you should contact a patent lawyer to provide you with a search report on your invention and patent. If you suspect that the co-owner has another application in the system somewhere, then the proper and complete search has to be done by a professional who can bring his/her experience to serve you well.
You did not indicate what foreign country you thought this application may have been filed in. A lot of unanswered questions.
This is not a legal advice as I do not have an attorney-client privilege with you. You should retain a lawyer before acting on any generally available advice.
If the "co-owner" was listed as an inventor on the "rogue" application, then you may be able to search various patent databases for his/her name and try to determine if an application was made in his/her name. Most major countries have online patent databases that you can search. I hope that you have some idea of the suspect country or countries to start.
Under U.S. law, the inventors must be listed on a patent application. If you did not invent the "tweak", then you shouldn't be listed as an inventor. Hopefully you have some type of contractual agreement or formed an entity with the co-owner and agreed that your U.S. Patent and any related applications would be transferred to you and your co-owner/the entity. I hope you have such an agreement.
You should confidentially consult with an attorney to discuss the particular facts in more detail.
Short Answer: You may need to wait.
A patent application, whether filed in the U.S. or or internationally, publishes in 18 months after its filing. So you can find out if a patent application has indeed been fled at that time. To find out any earlier may be tricky. you can start by consulting an attorney. Perhaps a letter from him/her will get you the result you seek. Note, however, that if your former co-inventor made a non-obvious change to the co-owned invention then that would probably qualify him as the correct inventor of the new patent. Good luck.