How can one know if some of the content presented by a paper is protected by a patent?
Patent law varies a lot by place, of course. But patents are specifically not secret information. Nothing in US law, I'm pretty certain, would prevent anyone from talking about a patent or its contents. You can't implement it, of course, without a license, but words are free. 

Don't confuse patent with copyright. The latter protects words, the former protects inventions. You can't operationalize the algorithm in a software patent, but some representation of the algorithm is actually part of the patent.

For that reason, you aren't abetting the breaking of a law by writing about it. Anyone who reads the patent can see the descriptions. 

However, if you are actually building things then you can run afoul of patent law since it isn't words, then. You are implementing the "invention" described in the patent. This is why patent lawyers earn more than you or I do. 

Depending on your location, that may be different. 

See: https://en.wikipedia.org/wiki/Patent_infringement