How can I check where my site is managed?
Go to a DNS checking tool like ozdns.com.au or whatsmydns.net, type your domain in and hit Report.
The 3 most important DNS records are:
This tells you where your DNS is managed. For example, if your nameservers shows ns1.auserver.com.au it means your DNS records are managed at that server. This often (but not always) indicates that your site is hosted with the owner of that server.
This lets you know the IP address of the server that your website is hosted on. If you use a tool like this ip Whois Lookup, you can find the company that owns the server in question. You may still be under a reseller of theirs, but if you contact that company they may be able to get you more information.
This tells you where your email is hosted. Common MX records are ASPMX.L.GOOGLE.COM. (Google) and *.outlook.com (Microsoft's Office365). You may have up to 5 MX records, however, all will point to the same servers. In a similar way, you can use an ip Whois Lookup to find more information about where your emails are hosted.
If you do a WHOIS lookup on your domain, this will provide you information about the company it was registered with, among other things. For '.au' domains, you can use the AUDA's WHOIS lookup instead. You will find the registrar name, and you can then search that company to get more information on where your domain is registered.
Note: Many companies use larger domain registrars like "TPP Wholesale Pty Ltd" as resellers. TPP has a provider checker so you can find your actual domain registrar more easily.