Going out to eat with food allergies can be an absolute nightmare. Even if you tell the place beforehand, you can never be sure if there are traces of certain things on their cutting boards, knifes, or in the air in the kitchen. This can cause serious anxiety and even more serious complications if you are very sensitive to certain foods.
A few tools that have worked for me and are at my disposal are found in the Internet and the modern age of allergy awareness. You can choose where you eat and there are certain places that do a great job looking out for you while others that are best to stay away. First, research can be your best friend. Before calling a restaurant, take a look at the menu. If you are allergic to peanuts for example, see if there are any dishes that stand out as maybe containing peanuts. If so, you know you are risking and may be able to make a more educated decision going elsewhere or talking to the staff.
Food allergies are no joke and the last thing restaurants want is an illness on their hands. The liability can be awful. That's why I always talk with both the manager and the waitstaff. I usually call beforehand when I am reserving my table so that they know that I am coming and can make any preparations in the kitchen. I also tell the waiter and the host when I get to the restaurant. This ensures there are many lines of communication between me and the restaurant in case there is a breakdown.
Lastly, if it is a place that I know will be sketchy no matter what, I always bring my own back up snack. It may seem rude, but trust me, when you know that you may be poisoned, it may be best to just munch on a pocket snack while others are having a good time. It's sometimes just safer than risking getting sick.