The only option for finding you if your wireless is as good as an cable connection is to test it out.

AFAIK if on your computer you get "Connection excellent" then there should not be any noticeable difference. Anything below that you will need to test by trying both with and without cable.

Concrete walls and similar obstacles have a habit of making the wireless connection drop to poor and then back to good etc.

In my case I can't use wireless for downloading (connection not good enough), but have almost no problems browsing the net and such. There is a concrete wall between me and my router.

BTW many wireless router have the option to change signal frequency which help a bit in some cases.