Wake On Lan behavior with Mac Mini - how to put asleep and confirm wake success?
To determine if a computer is sleeping:

Use hardware, such as the ‘Caps Lock’ led on a wired keyboard as a proxy for sleep, or the connection light on certain bluetooth hardware
Prior to putting the computer to sleep, check for potential ‘Power Assertions’ that could be blocking sleep
Review the Console system logs afterwards, looking for messages relating to sleep and wake events
Ping the Mac’s local network IP address from another computer - a properly sleeping computer should not respond (at least not in a timely fashion) to a normal ICMP ping (but this may not be decisive for recent macOS versions on recent Macs)

Things like music playback or open SSH connections are good indicators of wakefulness but not of sleep, partially due to #2 above.
The pulsing computer LED is a prima facie indicator of sleep.

Note: if you have certain other Apple devices on your network (certain Apple TV and iPad models, and maybe other Macs and Airport devices), the indicator in your WoL app is likely green because you have “Wake for Network Access” enabled.
These devices monitor the network for connection attempts to your sleeping Mac’s local IP or MAC address - if they see that network traffic, they will send a WoL packet and wake your computer (it’s sort of a proxy WoL system). In this case, your Windows WoL app (or another network device) could actually be what is unintentionally keeping your computer awake.