In case this is still actual, you can find some hints here: https://wiki.shibboleth.net/confluence/display/SHIB2/IdPDiscovery. on the Shibboleth Wiki here.
The key idea is to provide a entityID to the IdP in question in a manually crafted login request to the SP login module, like
https://sp.testshib.org/Shibboleth.sso/TestShib?entityID=https%3A%2F%2Fidp.testshib.org%2Fidp%2Fshibboleth
where entityID=...
is the url-encoded entityID of the IdP in question.
The documentation of this parameter can be found on the Shibboleth Wiki in the "Advanced configuration" section.