hi,

in fact, there is one more reset source - the PCA watchdog integrated in module 4.

As about reset source then only POF (bit 4 of PCON) may be used. It is set by hardware when Vcc rises from 0 to its nominal voltage. Then you should clear it and with this way you may recognize resets from Watchdogs and RST pin - they do not set this bit back.
I have not checked for does internal power monitor set this bit or not; probably you may check it with a little test yourself or ask Atmel tech. I hope that internal power monitor does set this bit because it seems logicaly: if power comes down of valid level then user must know about it. The reason is that after power failure, RAM content is indeterminate and you need to (re-)init it. In case of other reset sources RAM is not affected. It is why the POF has been implemented. By this flag it is possible to know about power failure. As internal power monitor checks for power then it should indicate about failure with this flag as well. But it is my opinion only; do test or ask Atmel and report here, please.

Regards,
Oleg