Seat Pitch is the distance from any point on one seat to the exact same point on the seat in front or behind it. While it is not the exact equivalent of "legroom", it does give a very good approximation of how much seat room you should expect. Bottom line: the more seat pitch the better.
If the airlines publish seat pitch or seat width information, it is listed on the website. Some airlines, however, do not publish these measurements, in which case they are most likely absent from the site. In this case, try calling the airline directly for additional information.