This - isn't a good sign. Aliens are shooting at something, a mile or two up the road. If you're real quiet, you can tell its them. Their rocket guns sound like fireworks - thoosh! Tok! Bam! They don't hit supersonic speeds instantly. You don't really hear lasers, there is kind of a sound like popcorn if it hits a soft target that immediately bubbles or cracks, but nothing like movie stock footage.
What does make a unique sound is the radiation gun particle beams - this loud constant crackling or saw like noise. Which strikes me as a bit odd - the X-ray tubes I worked with at the hospital way back when didn't do anything like that - a soft hum maybe - more likely a loud buzzer hooked up to the system to give an audio warning the machine was in operation. Maybe its the same sort of thing - an added sound device so no one accidentally goes waving around a gun shooting invisible death beams.
Of course, the big give away more often than not is missiles. Their supplies are a lot more reliable than accidentally wandering across a take that was slagged before it got off all its shots. Still, they seem a bit reluctant to use them now. Early in the occupation, they would use missiles to collapse buildings to deny their use to guerrilla fighters preemptively. Now they tend to wait until something is a confirmed threat, and after clearing it, use conventional demolition techniques.
We don't hear them complain about it as much, but it must be five years for them too. Maybe by now they'd be willing to talk a bit more. Not too sure how to do that though. I mean, you could just walk up, wave, and say hello. But its hard to tell if what you'll hear next is "and good day to you too" or a circular saw like warning buzz.
I suppose you could find them when they're engaged in combat, and shoot a few targets for them - prove you're on the same side. 'Course that also proves you're armed and a good shot.
Right away I can say a white flag wouldn't work. Its not like their signatories for the 19th century Hague convention on lawful warfare. They probably don't even know what a Hague is or where it is. Heck, I don't know where it is either.
Wait, wait. The noise is getting louder, the aliens are getting closer! What do we do, what do we do? Well, yeah, I suppose we could just play dead and wait for them to pass. Easy for you to say, you already are. But what about me? Never really made plans about what would happen if I ever found myself in a burned out APC talking to a bunch of skeletons about aliens. Didn't really come up when I was cramming for the medical exams. Mom warned me about this, but she was quite drunk at the time...