You don't start getting negative effects until your dosage gets into the hundreds, and you'll generally only get that by going to the Glow in FO1 or the Toxic Waste Dump or Gecko Reactor Room in FO2, or by fighting lots of floaters and centaurs. Small doses like that are generally from eating irradiated fruit or from the stray centaur/floater strike. If you're concerned, you can buy or find a Geiger counter, which will tell you your radiation level. You don't get ANY negative effects until it gets to 150, but anything past around 400 can kill you outright depending on your stats. It's a good idea to always carry some Rad-Away around with you, and it's pretty common as drugs go, so don't worry too much about wasting it if you don't have a Geiger counter on hand.
