Its just plain rude to plug into an amp without asking, unless they were kids and a bit naïve. Simple answer is, as has been said before is the organiser to sort it all beforehand. We are playing tonight and there are 5 bands on, with us being 4th on bill. The organisers have already set up back line and we've been told to just bring guitars and FX. This does however have a downside as I have no idea what amp I will be playing through and we have been told we must not bring our own amps,