I'll use the gnu find syntax for the -perm flag in this example:
Basically -- if you throw out wacky extensions like ACLs, you've got 3 chocies - owner, group, and "other" write access. Sounds like a job for a loop.
There is plenty of room to optimize this but I'll leave that to someone else... Also, I never can remember all the details of find and crossing filesystems and that sort of nonsense. Also, make sure the output of groups is the same as on my test linux system
$ groups snoopy
snoopy : snoopy doghouse linus admin wwI woodstock
This is a rough example of how you'd find files writable by a user. This will when run as any user, but if you run it as a non-uid0 user you'll only find things that are in directories that the user running the script has both read and execute permissions to.
# first files owned by the user and writable
find "$directory" -follow -user "$user" -perm /u+w 2> /dev/null
# now for files that are group writable with the user in that group
for groups in $(groups snoopy 2> /dev/null | cut -f2 -d:)
find "$directory" -follow -group "$user" -perm /g+w 2> /dev/null
# now for everything else
find "$directory" -follow -perm /o+w 2> /dev/null