How do I know if my ISP is doing that? And which ISP do I change to? Where I live, there’s effectively a single available ISP unless I don’t mind dropping down

How do I know if my ISP is doing that? And which ISP do I change to? Where I live, there’s effectively a single available ISP unless I don’t mind dropping down to single digit mbps. Seems more plausible to just run DoH for my DNS and then not worry about my ISP snooping on my DNS requests.