Unfortunately, this is only relevent if you actually KNOW "reality and actual physical law."
Yes, and we do, to a very large degree. Althought science will not stop discovering for a long time, much of what it has already discovered and explained will remain. We have all reason to believe this tendency will continue over the eons.
Contrary to common belief, science does not change completely form time to time. There is a hard core that stays and there are inconclusive shells that change (such was the nature of eletromagnetic phenomena in the 19th century). We tend to focus only on the edge of science and not on those stablished knowledges which we take for granted.
I cannot, of course, know beforehand how to specifically test any possible "alien technology" that could possibly show up. But they WILL BE testable. They will have to be matter of science, they will have to work in a manner analogous to any other machine or technology. If it is a remote mechanism, a communication medium will have to exist, and we will be forced to find its source. People in 1000AD would reach the same conclusions over time.
Eventually (5000 years? whatever), what is technology will be shown to be so. But what if it isn´t? Our efforts will be useless, there will be nothing to explain. We will be able to use extensive testing to rule out the "alien" hypothesis, just like lack of evidence allowed us to rule out so many other at-the-time good ones.
Now, I know many believers like to point out what they think are "paradoxes" of science and immediately say that "can only be explained by God". There is that whole woo in the University of Oregon. I´m not one of those people and I´m sure that they are speaking out of ignorance (or maybe out of their...). Looking for horns in horse´s heads, as we say in here.
I hope you understand I´m proposing something different altogether.
