I've seen recommendations to do a protein rest for under-modified malts. What are under-modified malts and how would I know if a malt is highly modified or under-modified. I had a quick look at this article but it's pretty technical and I didn't see how I could know before I purchased the malt. Are there any ways to know by reading the specifications - for example a page like this.
The article you linked to states anything less than 78% extract dry basis fine grind (DBFG) is substandard or under modified.
There is also this paragraph: Grind difference (% FG/CG). The fine grind/coarse grind (FG/CG) difference indicates the modification of the malt, and maltsters often use it instead of the DBCG value; either can be readily calculated from the other if the DBFG value is listed. A "steely" or vitreous malt, one suitable only for a mash cycle that includes a protein rest, will have an FG/CG difference of 1.8-2.2%, while a mealy and well-modified malt eminently suited to infusion mashing will have an FG/CG difference of 0.5-1.0%
That said, most all malts are very well modified today and have been for decades. Under modified malts are a problem pre-dating advancements in malting chemistry. It is so much so the norm, that if a malt is under modified it is usually labelled specifically as under modified malt and you don't need to worry about searching spec sheets for some magic stat telling you what degree of modification it is.
Modification is the process of barley becoming malt. It happens naturally by adding water which activates enzymes which break down a starchy endosperm inside the barley husk into carbohydrates. This is called germination. Depending on the barley variety and growing conditions, there will also be varying levels of proteins in the endosperm.
Carbohydrate modification (starches broken down into sugars) can be indicated by the difference in fine/coarse, viscosity, friability, and beta glucan level. A higher fine/coarse difference means poor modification, and higher friability means higher modification.
One measure of protein modification can be the ratio of soluble proteins to total proteins (S/T ratio). Also called a Kolbach index. This number can range from mid 30-45+ in fully-modified malts, but that ratio is dependent on the original protein levels in the barley so is variety dependent.
You can request a COA (certificate of analysis) for every batch of malt, which will have most of the above mentioned numbers on it. For example the specifications you linked to in your question have a range of possible values, meaning it really varies batch-to-batch and crop to crop. There is not a single number that says 'this is fully modified malt'. That being said, modern malts are all highly modified, and as far as homebrewing goes a protein rest for under-modified malt is not usually necessary. There could be other advantages to doing it, such as flavor or tradition.
Malting and malts for brewing can be a very intricate and detailed subject, entire volumes of books have been written. For a gentle introduction, I highly recommend the book Malt from Brewers Publications.