Determining if Server/Client's TLS configuration is Vulnerable to SLOTH
It doesn't seem that openssl is capable of doing this itself, especially against servers that don't advertise RSA-MD5 support but will still accept it (e.g. OpenSSL pre-1.0.1e).
But see https://securitypitfalls.wordpress.com/2016/01/06/testing-for-sloth/ which appears to provide a handcrafted TLS1.2 client that will test for the broken behaviour.  I haven't tested it personally yet, but it looks like exactly the sort of tool that I was looking for to perform exactly the testing you describe.

Thus, to test properly if a server is vulnerable against this attack, we need a client that is misbehaving.
For easy writing of such test cases I have been working on the tlsfuzzer. Just released version of it was extended to be able test servers for vulnerability against the SLOTH attack (to be more precise, just the client impersonation attack – the most severe of the described ones).