HI @Masii. To find the devices logged into your account you just need to go to spotify.com and find you account, go to the drop-down box and you can find devices and see what devices are on your account.
Thanks for reaching out to the Community. We're happy to shed some light on the situation.
While there is no page that will show you a list of all your logged in devices, you'll see an in-app notification when your account is being used somewhere else. We also send emails about new logins to help keep your account safe. If you see a login you don’t recognize, get help here.
In the past, it was possible to see all of your offline devices on your account page, but this option is no longer available. However, adding a new device when you've already reached the 5 devices limit will automatically remove any downloads from the one device that hasn't been used in the longest amount of time. Reinstalling the app will also remove any downloaded music from your device.
Keep in mind that if you'd like this feature to return, you can consider adding your +VOTE to this idea. The Community team is actively bringing your feedback to the internal teams at Spotify - check how here.
In the meantime, if you're having difficulties with your downloads for offline listening, we'd recommend to check out this support page.
Hope this helps. Let us know if you have any other questions.
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"Where words fail, music speaks." - Hans Christian Andersen