While I'm a lawyer myself, I'd respectfully disagree with your advice. Assuming that the original post is not a hoax (and, again, I reiterate my concern that it's a front page MO story that hasn't been verified in any way), I think it's totally unacceptable to file a wrongful death suit against the driver in the absence of some evidence of wrongdoing on his part. Undoubtedly some lawyer will be willing to do it, but imagine, if you will, the pain and anxiet that you'd be inflicting on someone who--as far as we can tell by what we know--did nothing wrong.
To the original anonymous poster--if this truly happened, there are specialists who can reconstruct accidents. The fact that you've written four months after the event doesn't help, but reconstruction experts can often determine if a vehicle was running or lights were illuminated in the course of a thorough investigation. The officers on the scene, hearing the truck driver's version of events, should have at least made some preliminary efforts to determine whether the bike was running--it might be as simple as looking at the position of the ignition switch and/or kill switch. In order to determine if lights were illuminated, the bulbs would have to be located and examined. You, as a non-relative would have limited abilities to gain access to the bike to allow an examination, but his next-of-kin should be able to do so. An examination of the accident scene would be very helpful to a reconstruction expert, but the passage of four months probably means that such an examination would be fairly useless. The photos you're referring to would, in likelihood, not tell an expert whether the bike was in motion, though it might be possible to estimate the relative speed of impact. The skid marks and debris at the scene of the accident would be much more helpful in this respect.
It's hard to imagine a scenario in which a rear-end collision of the sort you're describing would leave the motorcycle upright if it was moving at the time of the collision. Certainly the gyroscopic effect of a bike's rotating wheels would enhance the possibility of that happening, but it seems much more likely that a motorcycle moving at speed would be upset in its trajectory by being hit from behind and would have either been propelled sideways at an angle or have gone down--I'm not an expert, but based on what you've written, the truck driver's version sounds plausible.