How can i tell if the fish oil pills i buy are good and not contaminated? what is a good safe brand?

Molecular distilled: Good question. Fish oil can be contaminated with mercury, pcb's etc. A high-quality fish oil will say it has undergone "pharmaceutical grade molecular distillation" & independently assayed to find no detectable toxins. Ideally it will also be esterified with triglycerides to enhance absorption. Good brands include nordic naturals, designs for health, carlson & minami. Consider krill oil as well.