The only real test is to just use the service you’re trying to unlock and see if it works.
Note that having a residential IP doesn’t necessarily mean your VPN will not be detected. It only protects against simple IP lookups, not more sophisticated detection methods.
I prefer ipinfo.io since it also works through CLI (eg, curl ipinfo.io) so I can test if the browser use its own proxy. If I just need the address, icanhazip.com, I can also force it to try ipv4 or ipv6 through ipv4.icanhazip.com and ipv6.icanhazip.com to check the IPv6 support and priority.
I’m not understanding why you would need to check? Are you concerned someone upstream is proxying your traffic through a VPN without your knowledge? Since you say this is a residential connection, do you not have control over your internal network, and are concerned your ISP might be vpn’ing you without your permission? I’ve never heard of an ISP doing something like that.