This is an interesting question because there a some people that believe that the amount of protein breakdown that can happen in a protein rest is next to none. The reason being is that the enzymes are very heat sensitive and are denatured during kilning. The people who think this believe that the rest is important for the breakdown of beta glucan (the cell wall material that surrounds the starch). This beta glucan is very viscous if it is not broken down and this rest helps with breaking down these materials. This is why John Palmer says to do it if you are using roasted barley because that is unmodified barley and has all of that beta glucan present.
That being said, what I just stated about protein breakdown isn't a proven fact and is not accepted by everyone (most German brewers believe that there is protein breakdown at this stage, as well as Dan Gordon of Gordon Beirsch- who studied in Germany), but you will break down some beta glucan which will decrease the wort viscosity(making lautering easier). However it is under debate whether or not there is a breakdown of protein as well.
Originally Posted by hector
The best way is asking the malt producer about it .
I take a randomly amount of the pale malt and look at each seed carefully to see the length of the Acrospire underneath the husk .
The acrospire length is a measure of modification and a well modified malt should have 75-80% of the malt have acrospires that have grown to 75-100% of the length of the grain.