I would start by doing a general internet search about the food item I heard was potentially dangerous. While doing the search I would first look at any recent news articles related to the food to determine if it was potentially dangerous, and I would make sure those news articles were reliable, I would also look to see what research or experiments were done to determine that the food was dangerous. Next I would look to see what other articles on the internet had information about the food being dangerous I would also look to see if there were any articles that said to be on the look at for a scam or false news report about the food being dangerous I would look to see what kind of research was done because if only one experiment had been done to determine the food was dangerous then I would not feel like that was sufficient evidence to make such a bold statement The participants in the experiment could have just been allergic to the food so that would not mean the food was harmful to anyone that ate it.
You would also want to look at the statistics because if only a small percentage of people were negatively affected by the food then I personally would not feel like those results were a strong enough factor in making a decision to quit eating the food.
You must also make sure you look for only reliable sources.
This would be sources such as accredited organizations or medical research facilities that can be trusted not to make outlandish statements which are not true.
Once I came to a determination I would start by simply sharing the results with people I know. If the advertisement was false, I would begin telling the people I know that it was false and how I came to that result I would also probably share any articles I had discovered in my research on social media or blog forums so that other people could have access to them and could read them their selves so they could see the proof 1 had gathered to form my opinion I would also feel obligated to try and figure out a way to contact the television channel that aired the commercial to inform them that the commercial they were airing was false or didn‘t have sufficient evidence to back up the statements I am not sure if they would decide to take the commercial down because of that, but I would feel better knowing I had done everything in my power to alert as many people as possible to the truth.