In May 2019, World Rugby published guidelines for dealing with high tackles and shoulder charges.  The framework document lives here or in the pdf attached.  WR says:

Under Law 9.11, the referee is always entitled to issue a red or yellow card for anything that they deem to be reckless or dangerous.

However, the following guideline is intended to improve consistency in application of sanctions by distinguishing between dangerous tackles that warrant a penalty, yellow card or red card. The framework also supports protection of the head of both players by consistently and frequently sanctioning the tackle behaviour that is known to be the highest risk.

I suppose the first thing to look at is the laws.  World Rugby posts the current laws along with a lot of ancillary information on their site.  https://laws.worldrugby.org/ 

There's also the Playing Charter and Foreword (really, read them), application guidelines, law clarifications, referee signals, variations, and something called XRugby that I have no idea what it is.  You can also get the laws (but not most of the other stuff) in an iPhone app.  They might even still print the laws in a little book, but if they don't you can print it yourself from the pdf below.