Well, you probably won't know directly. If you suspect you have a problem, you can monitor the Security event log and use a tool to analyse the web logs for attacks. But if you follow reasonable security practices, you probably won't have too much trouble. You should forward the minimum number of ports (which would be 443 and 4125), and use strong security for those users allowed remote access.
The certificate has two problems. First, it's self-signed. Second, from anywhere outside your home network, it's going to look like it's assigned to a different machine. We are assured that the cert issues will be solved by the time WHS is released.