How do you know that Npcap is installed

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Look at the text on the Wireshark Help -> About Wireshark -> Wireshark tab, or from the command line with wireshark -v or tshark -v`.

Whichever method is used, in the "Running on ..." section you will see what capture library is being used, e.g.

Running on 64-bit Windows 10 (2004), build 19041, with Intel(R) Core(TM) i7-8700 CPU @ 3.20GHz (with SSE4.2), with 65342 MB of physical memory, with locale English_United Kingdom.utf8, with Npcap version 0.9996, based on libpcap version 1.9.1, with GnuTLS 3.6.3, with Gcrypt 1.8.3, with brotli 1.0.2, with AirPcap 4.1.3 build 3348, binary plugins supported (0 loaded).

You can also use the OS features to list installed applications e.g. "Apps" on Win 10, or Program And Features" from earlier OS's, or you can go grubbing for npcap binaries, usually in C:\Program Files\npcap or look for the driver in <windir\System32\Drivers\npcap.sys.