"The Core" (c) DigitalBlasphemy.com
OK, so how can we speculate, predict? One aspect is easy - the laws of physics, including the nature of space-time, are universal (or, at the very least regional). We can make some predictions based on predictions we can make for our own technology (for instance, the development of materials that can hide objects from light and sound).
But after that, much of alien technology will be influenced by the alien's 1) biology/physiology, 2) sociology/culture, 3) theology, 4) physical environment (planetary as well as the space environment), and 5) any outside influence (from another alien culture).
And as we will see later, all of the above are somewhat interconnected - each influencing the other to some degree.