After posting, I noted I had been beaten to it by ORAC, with a better response than me, but I'll leave this in anyway, if for no other reason than the link.
Rules of Engagement (ROE) are many and varied, and apply in the air just as on the ground. However, they only apply to those who follow them - that is to say, if NATO, for example, is faced with a threat, its response to that threat is determined, in large part, by what has gone before and what the politicians agree. Their decisions are based on political intent, but also advice they receive by an army of lawyers. However, not all potential adversaries (countries, factions, terrorist groups) follow such things as ROE - or indeed recognise the authority of international courts.
Returning to your original Q, I don't think it's terribly likely that a western power would authorise engagement of an ac without visual inspection
unless it was part of a larger conflict that had already developed, or there was extremely reliable intelligience (oxymoron?) that the threat was such that immediate long-range engagement were necessary.
The thing is, these days, it may not be necessary to visally ID a target to establish the type. Non-cooperative target recognition (NCTR) may give a level of assurance of the type such that engagement may be authorised without visual ID. More info here:
NCTR The other thing to bear in mind with ROE is that the right of self-defence always exists, and you may respond if you or a friend are under attack. Long-range missile launch indication against you or a friend may be enough in itself to justify a long-range response, even without NCTR.
Sorry, I got a bit carried away there - this really is a massive subject, and perhaps I shoud just have said
'No, not always!' STH