Look in the examples that come with FMP12: Contacts file
Although not visible on the layouts, in the scripts you will find an option to do a reverse geocoding using CustomFunctions that are also in that demo file.
So, even a limited address may give you geocodes, then use the geocodes to get back the full address.
I'd also suggest that you look at the documentation for the Google Maps geocoding API. I found while implementing geocoding for a solution that when you pass Google an address to ask for the coordinates, Google's response (in JSON or XML, whichever you're more comfortable with) includes the original address split out into separate fields and with normalized formatting like standardized street name abbreviations, and often including fields that may not have been in the original request, like the postal code. So, if you have a partial address and you want a more complete/corrected/standardized address, you only need to make the first Google Maps API call.