To find the DNS provider of a domain, you need to find the authoritative DNS servers for that domain (that is, the servers that are used to keep DNS records for this domain).
A list of DNS servers authoritative for the domain is shown in the Name Server (NS) record. To find this record, you can use the NSLOOKUP tool. Read the Knowledge Base article on How do I use the NSLOOKUP tool provided with Windows? for more information.
You can also use Who.is or any other website which provides the same functionality.
Read the Knowledge Base article on How do you know what my DNS records look like even though I don't host my DNS with you? How can I see what the DNS records for my domain name look like? for more information.
If Intermedia is your DNS provider, you should get the following Name Servers:
If your account is located on EXCH500 (UK) it should be: