There’s no inherent way to know exactly what client a validator is running. Researchers use other metrics to make deductions on which client a validator is most likely operating. The problem is they cannot distinguish with 100% certainty which client a validator is running.
Blockprint - Developed by Sigma Prime’s Michael Sproul, Blockprint analyzes each client’s block proposal style as described in this Twitter thread (Nitter).
Node Crawler - Developed by Ethereum team, is used for crawling network nodes and gathering the client data. Please note that it collects data based on number of physical machines, not validators themselves.
This means that this data should be used only as a guideline to the overall client sentiment on the network, not the actual validator count.