The last 4 digits in the Hull ID determine that.
Example ABC12345C393
ABC would be the manufacturer ID Code
12345 would be the hull serial number
C would be the month (March)
3 is the year built
93 is the models year
http://uscgboating.org/recalls/mic_database.htm
The above is can be used to find the manufacturer from the MIC code (first three digits)