These below recycling symbols on the food container indicate the type of plastic, but are not definitive of microwave safety.
Type 1 - polyethylene terephthalate (PET)
May be safe if marked "microwave safe", although some recommend against food contact when microwaving.
Type 2 - high density polyethylene (HPDE)
May be safe, although some recommend against food contact when microwaving.
Type 3 - PVC, polyvinyl chloride, vinyl
Do not use in microwave. Also, PVC often contains bisphenol A (BPA). Since this material is used for some plastic wrap, unidentified plastic wrap should never be used in a microwave with food!
Type 5 - polypropylene
May be safe, although some recommend against food contact when microwaving. "Type 5" are the most commonly labeled "microwave safe". Despite this, I have observed "Type 5" containers with partially dissolved surfaces, apparently from microwave use.
Type 6 - polystyrene, styrene, polystyrene foam
Not heat stable. Do not use in microwave. Do not microwave food in a styrofoam  container! In addition to not being heat stable, polystyrene is a potential human carcinogen and usually contains bisphenol A (BPA).
Type 7 - polycarbonate; "other" (can contain bisphenol A (BPA); most polycarbonate contains bisphenol A)
Do not use in microwave. (Note: Polycarbonate nursing bottles which have been boiled or washed more than 20 times or are badly scratched should be thrown out.)
Paper - food safe but Avoid using paper coated with plastic in the microwave.
(Some types of paper may catch on fire under some circumstances. Some sources suggest avoiding using newsprint. Paper which is not food grade may also include toxic inks.)
These informations are cited from Seattle Community Network