Hi nicolac, welcome.
Quick answer, yes. Long answer, it depends.
Please tell us how you want to do this. If you can scan or photo the stamps and then post the pics here that will help, depending on how many you mean by big bag.
Or do you want to have someone look at then in person, or do you wish to take them to someone personally?
Generally, stamp condition determines its value. Rarity is of course a factor too but most stamps are not rare.
Are the stamps loose or attached to pages or in stock sheets?
To get the best response on this forum please post questions and scans of one country at a time in a separate topic. That way people who can help with that area will more likely see it sooner or later.
This forum has a photo/scan file size limit of 90 KB so if your pics are going to be bigger than that please use an off-site photo hosting service like imageshack or photobucket and then post the pics from there.
If you feel ambitious and want to learn or start collecting yourselves then try a stamp (philatelic) shop or stamp show or even a library to look at stamp catalogues there.
Catalog value is what dealers have recently sold these stamps for, not actually what they are worth to you, unless you plan to become a dealer yourself. Depending on condition of the stamp (wear and tear) you will get 10% up to 50% of catalog value, unless you are a super salesperson yourself and want to spend some time selling the stamps off on
ebay or at yard sales or flea markets (which does work).
We all like to see stamps so don't be afraid to post lots of pics.