I've gotten my best constructive criticism from annual entries into the AHA National competition. Also, in my homebrew club, if you can find a person that is certified and you know has a good set of tasting skills, I'd rely on that person. I've been lucky over the years to have a few people like that in my club. (salute George out of Mandeville, where ya at?)
A constructive review isn't "talking down" on a beer. I taste many beers at my meetings I'm not fond of ... but I don't say "bleh" instead I might point out any glaring flaws or possibly an infection etc.
I think a good start is to enter the AHA Nationals this year. Pick two styles that you consider to be your best and go for it. You get written reviews back and the beers are reviewed by several judges. The further it goes in competition, the more reviews you'll get. And heck, if you get a medal, then you know it's a good beer!