I'm an old user of Gmail since 2004 and I rarely have checked spam label in search of false positive spam emails.
Do you have some strategy/technique to find good emails or you just browse the spam label and check them one after one?
just browse the spam and check.. you'll get the hang of it pretty fast.. normal emails usually don't have many numbers and strange characters like % or UPPER CASE PRIZES etc. in the message preview.. so you can actually browse very quickly..
what I guess you're looking for is an automated way to do this - which would actually be a spam filter - filtering gmail spam filter..