First, you can never be 100% sure about anything security, it's all on a scale.
As for SSL, that just means your traffic from the site to your browser is encrypted it does not mean that you are ever secure, it is just a tool -employed properly and it does a great job (not perfect, but great).
Now as for MiTM attacks... well... unless you are the network administrator actively monitoring network traffic, it is very difficult (and sometimes next to impossible) to detect a MiTM attack.
That being said, you could always try a few things. First, run BlackSheep. It is a detection tool for FireSheep. Or use a VPN.
Personally, I just would not connect to an open WiFi, just use your cell to to anything important(ish).
Oh and also, Facebook is not entirely SSL (their profile images for instance are still unencrypted) so you are not even fully encrypted there...