Try and find the recycling number on the plastic somewhere its a triangle of arrows with a number in it. That will tell you what the pastic is made from....and decide from there....if theres nothing on it then assume its a mixed bag #7.......
I found this on the net on the numbers:
"plastics #2, #4 and #5 are generally considered safe. Plastic #1 is safe too but should not be re-used due to the risk of growing bacteria. Any other plastic should be used with extreme caution, especially around food or drink. The risk is even greater when heating food. For microwaving in particular, remember that microwave safe containers aren’t necessarily healthy. They just won’t melt. In general, it’s better to avoid microwaving plastic entirely and stick to glass"