Booking a flight with a luggage limit via Adioso / Expedia

Every airline I've ever looked at makes their fees and charges pretty clear somewhere on their web page. For example, here's AirAsia's Fees and Charges page. That's because they generally operate in at least one country where it's legally required. I suppose a really small airline might get away without doing that. Just searching for "[airline name] extra charges" normally brings up the right link in the first page of results.

Alternatively you can pretend to book the same flight on-line and go through the extra charges pages to see what they are. This will often highlight the cost of things after booking as it'll say "Save X% by booking extra luggage now rather than adding it later", or something similar. There was one airline I booked with who did a really shady thing and made you put in (and authorize) your payment details before showing you the 'extras' pages but that was a while ago and I can't remember which one. If all else fails phoning, e-mailing or contacting the airline via social media will probably answer the question.

There's no definitive global list of charges per airline that I can find, it'd be difficult to keep up-to-date and so you'd want to resort to the methods above anyway. However, the Civil Aviation Authority has a table for Europe. For the major US carriers I found a site called Airline Watchdog. I can't find much for other areas of the world.

In general there's a limited number of things you need to worry about, assume that low cost airlines will charge for food and drinks on board (although legally on medium to long haul they may have to provide a free meal, depending where they're going from and to). They may charge to choose your own seat, they'll charge for luggage, and there may be other small charges (Ryanair has so far not managed to charge for using the toilet, and I strongly suspect that was only ever a "it doesn't matter as long as they're talking about you" publicity stunt).

Watch out for extra charges depending on your payment method, these are often not revealed until the very end.

However, in most cases it's in these airlines interest to get you to pay the money up front (less hassle and annoyed customers in the airport, plus they discount it to make it look like a good deal). So they're generally pretty open about the charges on their websites.