This is one of the toughest things about ski flying. I use a real simple test. If there are trucks and SUVs driving on the lakes, it is okay to land there. If a lake is too remote to have people driving on it, you just have to figure it out some other way.
There has been much written on the site about ski ops that is worth reading. Mike Vivion, who had 30 years of ski plane experience in AK can save you some tuition, if you read his stuff.
Ice thickness, while critical, is not the only thing to worry about. Overflow will cause you a lot of problems. Often, when there is a snow load on the lake, it will push water up and will create a layer of slush between the ice and the surface of the snow. On really cold days, if you land in this stuff, you can freeze down, creating a very difficult situation. Springs, sub-surfact flows, ice movement, snow load and who knows what else are all potential contributors. If you are not sure, be sure to drag your landing strip first, carrying plenty of power so you can takeoff if it turns out to be slushy, then come back around and land in your tracks. If they turn dark, stay the heck away.