You can't kow, actually.
The only thing you know is that 99% of the time an amp outputs a much lower wattage than the nominal value. Most often 5-10 W is what you hear.
But, on clean tones the risk of blowing the transducer is real if you play very loud and you produce impulsive notes (like slapping, for example).
Lunchbox users that don't want to buy an external amp could consider a lower-wattage retrofit power amp. If the difference between the power amp's nominal power and the cab's acceptance is nevertheless huge, the only alternative to playing at careful volumes would be to use an external attenuator, like we used to do in the old times to have a distorted sound al home volumes with our tube amps.
This means taking more stuff with you and a more articulated rig tho.
Note: playing at careful volumes means (if your cab is 60W) to play as loud as a 60W amp would: not exactly bedroom volumes 
