For the most part.
The big problem is all the manufacturers have a different way of measuring their own product. And most of them find some weird test method to inflate the numbers.
If you see an independant review, (like here on Anandtech) that gives numbers for brightness and contrast, they are probably more accurate and honest than the advertisers claims.
Just look in the "Displays" tab at the top of your screen. They use a device called a "Spider" to get light readings from every monitor they test.