You need to understand what type of stuff you have before you dispose it off. However, you can’t know it until someone who has been in possession of that material since its inception or you’re the one owning it for years. If you are unable to track the history of the material then consider it a very dangerous material.
Take the diesel fuel to the laboratory for testing and ensure that it can’t be used in the vehicle with diesel engine. If it is crystal clear without any problem, you can use it in the car or sell it for few bucks otherwise you have to take care of it before disposing it off.
Call your state’s trash company and tell them about the material you have in possession. Ask them the possible solutions for the method of diesel fuel disposal and learn the hazards of keeping such a material intact as well.
Search in the phone directory to find a number for a company that is licensed to collect hazardous material. You might have to pay some money/fee to the collectors but that will help you save a lot of hassle of disposing such a dangerous material yourself. The potential damage that can be caused by diesel fuel if it catches fire will be far more superior to what you will be paying the collectors.