Thank goodness for the internet. When buying a car, whether new or used, you can quickly look up current or past models to see what their safety ratings are. Safety should be of concern to the prudent; in America, you've got a 0.5% chance of getting into an accident each year. If those odds sound slim, consider that that's a 1-in-200 chance. It's not highly unlikely that someone could crash into you and vice versa.
Even if you purchase a car with good safety ratings, it's helpful to consider how those ratings are arrived at. Organizations like the IIHS (Insurance Institute for Highway Safety) and the NHTSA (National Highway Traffic Safety Administration) calculate the ratings by crashing the car into static barriers, like concrete walls and fixed poles. In other words, it's a car crashing into something that does not push back, so to speak.
This means you can have a smaller vehicle with good safety ratings, and a larger vehicle with the same safety ratings, all calculated by crashing into these fixed barriers. But if the smaller car crashes into the larger vehicle, the safety ratings can be mooted because mass comes into play. In a head-on collision, for instance, the larger vehicle will force the smaller vehicle backwards, delivering much more energy into the cabin of the smaller vehicle. This is typically bad news, as you can see in this demonstration from the IIHS: