Although there are markings in form of symbols to tell us if a container is microwave safe or not, I have some plastic containers which don't have any symbols but are microwave safe. Is there any method or rule of thumb to find if a material is safe to put in the microwave?
The standard method when I was young, and microwave ovens were new to the consumer market (i.e. had only recently started costing less than a used car), the standard test of any cookware was to fill a glass with water, and put the glass and the piece in question in the oven for a minute. If the water got hot, and the cookware didn't, it was microwave safe. If the cookware got hot, it wasn't microwave safe.
Later, there was a gradation -- some cookware would get hot in the microwave, either intentionally (to provide browning, make popcorn pop, etc.), or otherwise, but was still safe to use, providing the cook knew it would get hot. Many ceramics are in this category; they absorb some energy and get hot, but aren't damaged by it.
Generally, if you have an item that is likely to be damaged by excessive heat (like most plastics), you'll want to check it using the glass of water method (be careful, the water will be hot, whether the item being checked is or not). If it's not prone to heat damage, like ceramic, you can just proceed to cook with it, being cautious of the potentially hot surface.
It is worth noting here (thanks for the prompt, @EdGrimm) that many plastic storage containers may soften in the microwave even if they test as "safe" on the basis of direct heating -- and even if they're safe with boiling water, as microwaves can superheat steam or water well above boiling. Beyond that, there is considerable evidence that heating any food or drink, by any method, in a plastic container of any sort is inadvisable due to the presence of chemicals like bis-phenyl A (BPA) and other plasticizers or manufacturing aids.
Very simple...After 10 seconds in a microwave oven if it melts or deforms in any way try another test victim. If it is hot to the touch do not use.
A suitable container needs to be transparent to radio frequency energy. if it is relatively cool to the touch that would work. Some plastics have impurities that could leach into your food as well as causing hot spots that could cause a fire or arcing inside the oven (especially if the impurity is metallic in nature).