Unknown spills are actually very common. Negligence is the cause of many hazmat incidents. In addition, the paperwork may not be available (If the SDS is in the cab of a truck leaking chemical I'm not going to walk up and get it), and the placards may not be visible from either a vapor cloud or the position of the truck. There are ways to figure it out, it just may not be initially available.
actually no by the way I am a firefighter-EMT in real life so I know this one because I specalize in hazmat, in fact you will be walking up to get it with SCBA on (and in a hazmat suit if you department supplies it of course) and you will have to get it because it is vital to get that information in order to know what to do, and actually we have a way to identify a chemical just by looking at the type of trailer it is, we just look in our ERG under trailers and we can find every single chemical and what it is and how to respond to it. If it's showing vapors then its a gas, if it's spewing out without vapors then it's most likely a commercial chemical. Lastly paperwork is always available because by law you have to carry paperwork with you when you handle the chemical it's law sorry but it is what it is that law was made to help us out when we are responding to this stuff.