I usually look for reviews online (like here on Android Central), followed by checking the most recent user reviews in case the app has specific problems with my devices.
Some apps come from LifeHacker or occasionally One Thing Well and Betalist. Mostly the app recommendations come from Android Central and then Android Community.
I discover apps by scrolling, really i never knew they're were such great apps that don't get they spotlight they deserve, I would love Tablified for my nexus 7!
I'll take a copy! Really the only way I currently find tablet apps (besides AC recommendations) is from the featured tablet apps in Google Play, but it's very limited. Been looking for a better option!