Blown speaker symptom

This is another thread related to the recent purchase of an SM900 head from SWR:

Some of you must have had this problem: I prefer to use my 900w head in bridged mono into my 500w rated ampeg 410 HLF because of the improved headroom and tighter bottom.

How do I know how loud not to go?

Or in other terms is there a way to know when you are blowing your speaker BEFORE actually blowing them?

(also does anyone know what footswitch to use with it, at the moment when I use the morley fsw I have one of the 2 EQ doesn't work...)

thanks in advance.