You can start by pinging your target system.
Take a look at the returned TTL value. As long as you’re able to ping the host directly without going through dozens of hops, the returned TTL should give you a hint as to what OS the system is running.
- Windows by default will return a value near 32 or 128.
- Linux by default will return a value near 63 or 64.
The TTL value will decrease by one for every hop that it takes. You can always run a
tracert against the target to make note of the number of hops if you’re ever in doubt.
You can find more details of other operating system at https://subinsb.com/default-device-ttl-values/