Final answer:
One way to verify if the yellowish vapors emitting from a cylinder contain chlorine is to bring a chlorine monitor into the vapor cloud to check the reading.
Step-by-step explanation:
To verify if the yellowish vapors emitting from the cylinder contain chlorine, one of the ways for an operations level responder would be to bring a chlorine monitor into the vapor cloud to check the reading. This method directly measures the presence and concentration of chlorine gas in the air. It is important to note that entering the hot zone to read the label on the cylinder might be dangerous when dealing with hazardous materials. Contacting CHEMTREC for assistance could also be done, but it is not specifically mentioned as the correct answer in this scenario.