The EWG (Environmental Working Group -an independent entity) puts out two lists every year: the 'dirtiest' and the 'cleanest' fruits and vegetables (and you need to check them every year because a veggie or fruit can go from one to another like tomatoes did one year).
The '12 dirtiest' lists the ones that should always be bought organic while the '12 cleanest' is the one that lists the ones that you know are safe to buy 'regular'. I buy the 'dirtiest' always organic and, if I find anything that is not on it at a good price, I get that in organic, too (and don't forget to check what they call the 'dirtiest plus list' which elaborates a bit more and adds two more to the list)
Here it's the list of all the produce they tested for 2014:
http://www.ewg.org/foodnews/list.php (it goes from worst to good so the number one is the dirtiest -apples are ALWAYS no. 1 because they show more than 40 different pesticides, fungicides and antibiotics on them -and strawberries are no.2 so they are pretty bad, too)