I'm with you. The only hint is that in Portuguese almost every word ending with -o is male and ending with -a is female, with a few exceptions. Additionally, most of the words in Portuguese have the same noun as in Spanish. Exceptions are more frequent, though. By the way, I googled about general gender rule in Spanish and found this: http://spanish.about.com/cs/grammar/a/genderreversal.htm. As far as I read, they are also true in Portuguese.