How can one check if the food does not have excess levels of pesticides?
The need of the hour is to have some machine/tool that checks what is the composition of the food that we are eating and advices us to stop or go ahead with it. I am not sure if there is anything created like that? To take the examples of the news: Salmonella in eggs
How can we ensure that the food that we eat is not unhealthy?
Practically, you can't. You should clean produce appropriately to remove surface pesticides, and cook food thoroughly to prevent illness from food-borne pathogens.
Checking for levels of pesticides or presence of pathogens would have to occur in a lab, involving microscopes, chemistry techniques, and bacterial cultures. This is impractical to do in the home, plus it would likely destroy the food you were testing.
http://www.psfk.com/wp-content/uploads/2010/04/heidi_kenney_organic_food_chart_1.jpg
This picture is a quick and dirty cheat sheet that shows which foods generally have the most pesticides used.
As far as meat goes, most of the illnesses are byproducts of factory farming methods. Beef should ideally be grass-fed and pastured. Cows don't naturally eat grains so the corn-fed varieties have a skewed omega-3/omega-6 fat balance. Corn-fed cows also have weakened immune systems which necessitates the antibiotics that many people try to avoid.
www.eatwild.com has a list of farms in your area that support pastured livestock. It will generally be more expensive than store bought products, but if you are looking for higher quality food, it is the way to go.
Some foods are known for risks of unpleasant germs. These foods (like eggs, fish, meat etc.) should be eaten cooked.
Regarding pesticides and such, there really isn't a good way to get rid of them, and if you're really worried, you need to find a trusted source of organic (unsprayed) vegetables and fruit. This is probably easier than you think, but there is usually a price premium involved. These days, most commercial pesticides aren't terribly toxic and are probably on the same level as food colouring. Best avoided, but not exactly lethal.
As with most other things, the solution with pesticides is prevention as opposed to treatment. That is, avoid rather than clean off.
