I usually search on youtube for some reviews, or on google, remember that you need to keep in mind that a crappy guitar on a big amp with lot's of effects can give you the illusion it sounds good in a video-review

I also use ultimate-guitar.com but there are a lot of kiddo's on there shouting around

I might be searching for a new acoustic guitar soon and I have no knowledge about acoustic guitars whatsoever so it'll be a long search.. but take the time and eventually you'll find something that fits