Personally, I do this type of research using a number of different publicly available "route servers" or "looking glasses." Simply do a web search for "public bgp looking glass" and you should find a significant number of results.
Many of these tools now have a web interface you can access, although some are still network devices you can telnet/ssh into to run queries directly (if you know what you are doing). It would help to have some basic understanding of BGP command output, but for a number of tools, this isn't necessary.
You will need to determine an IP address used by the ISP network or the BGP ASN number of the ISP in question. Using the BGP routing information, you should be able to determine if the ISP has multiple peers. Each tool will likely work a bit differently and will often give you different results based on their own "unique" viewpoint of the Internet (i.e. where they are connected, where the target is located, which networks exist between the two, etc).
As an example, you can use the Hurricane Electric Looking Glass website to perform a query based on an IP address. Select a source router, choose the "BGP route" command, and enter the IP address. The results should be displayed in a table after you press the "Probe" button. In the "Path" column, should be the ASN path to the destination IP.
Clicking on that ASN get you further information about the AS. This information is contained in a number of tabs, the ones you are most likely interested in would be "Peers v4" and "Peers v6". This will provide a list of what HE has determined are the peers of the AS in question.