The only thing Google can do is rely on certain metrics, it's basic data analysis.
If you're buying links from a website that sells a ton of them, in an obvious way, then there is a high probability Google will catch on to that, sooner or later. Webmasters that sell a lot of guest posts have already been getting outgoing link notices, this is nothing new. The algorithm is going to get better at detecting these types of paid link schemes.
If you're careful in your selection from where you buy links then Google will never have a way to tell if a link is paid or not. Basically, anything that is high quality and mimics a natural link, and is not sold in bulk, is going to retain its value.
You just have to not buy links from garbage link farms. What is identified as a link farm is what Google is going to work on and they will get better at detecting these. It will never be 100% accurate, and as long as you buy HQ links, you will surpass this filter.
Remember that links are not the ONLY ranking factor (like they once used to be) you also need good content. Focus on good content and good links. You do not need a ton of links to rank, so do not overdo this. High quality, fewer links, better content that addresses the search intent.