Multiple admin panels and how to access all

I did not set up my sites. So, I can’t find where to access all my admin panels for the different domains that are now using the same theme. I can access one admin panel that my tech guy gave me a direct link to : IE http://domainname/wp-admin – and i have full access to that one panel. However, i need to konw if the other sites that are on separate domain names will have to be accessed outside with a different user and password. If so, i don’t have that.