On the most basic level: do an ls -l on a file (ls -ld on a directory only) to see which user and group own the file/directory and to see the permissions on the file. File permissions look something like this:
-rw-r--r-- 1 root staff 231 Oct 17 2001 weird.txt
In this case, root is the owner, staff is the group. permissions are read and write for the owner, read for the group, and read for all others.
If permissions were -rwxrwxrwx, the file would be wide open for everyone.
if they were -rw-r-wr-- and the user belongs to the staff group, the user would be able to both read and write to the file. So, you need to know which groups a user belongs to to get the whole pictre.
You can also set up ACLs to grant permissions to file.
Is this what you were looking for?