Antenna gain is a specification published by the reputable antenna manufacturers. You'll have to hunt for the information in some cases, other manufacturers make the information easier to find.
Each of the following vendors offer gain specs in one format or another:
Antennacraft:
http://www.antennacraft.net/index.html
Antennas Direct:
http://www.antennasdirect.com/
Channel Master:
http://support.channelmaster.com/att...+-+25OCT11.pdf
Winegard:
http://www.winegarddirect.com/
There are other manufacturers or brand names that have some known good product despite the lack of published technical specifications.
Foliage and building penetration loss is often variable with weather, wet roofs, siding and leafs are more trouble than dry. The best advise is avoid mounting an antenna where it is blocked by vegetation or structures. Presume metal to be impenetrable by TV signals.
Looking back at your previous posts, I see you have or had a Winegard HD7698P. The gain specs are at the bottom of this page,
http://www.winegarddirect.com/viewit...P%29&p=HD7698P