Go to ipleak.net and activate the torrent check, paste the magnet link into your torrent app while on VPN and it will tell you what IP address it detects, if it's not the same as your regular ISP then you can verify that it's using your VPN and not your standard internet connection.
On an unrooted phone, you would have to use block connections without VPN. That's the only way you can tell for certain that traffic is going over the VPN and that the VPN torrenting connection is not intermittently dropping.
If the device is rooted, then you could do actual packet analysis via tools like tcpdump to actively check the traffic is routed to the tunnel.