Postdocs are more or less government employees and so in a lot of EU countries, postdoc salaries are publically available so a little bit of digging will usually tell you the exact salary of any position you see advertised.
However, if you only want a quick rule of thumb to compare different European countries I would go with nominal GDP per capita. Yes, cost of living is higher in the wealthier countries as well, but academic salaries usually overcompensate for this (when comparing countries in Europe). The standard of living a postdoc salary buys you in Switzerland or Norway is much higher than in Greece or Portugal.
If cost of living differs a lot within a country the academic salaries usually do not fully account for these differences so living in an expensive city will make you relatively poorer.