Overstock.com has provided an outline for how to determine if your bowls and plates are microwave safe, especially if your bowl or plate does not have any indication on the bottom of the dish.
Many bowls and plates are considered microwave safe; however, if you use the wrong dish, you could ruin both the dinnerware and the dinner. Microwaves are convenient appliances, but there are a few precautions you should know about when it comes to your dishes. Always check to see if a dish is microwave safe before using it in the microwave. Some containers can break, melt or scald you. Some containers can also damage your microwave, such as foil-lined containers. Microwave-safe dishes can be used to both cook and store food. Non-microwave safe containers can only be used for storage or serving
1) Examine the Dish
Turn the dish in question over, so the bottom is facing you. Read the back of the dish. Many dishes and containers state either "Microwave Safe" or "Not for Use in Microwaves." If this isn't clearly stated, continue to Step 2.
2) Turn Up The Head
Set your microwave oven to its highest power setting. The method varies based on microwave manufacturer. Consult your microwave's manual for exact instructions. Most microwaves have a "Power," "Power Level," "Menu" or "Settings" button. Usually, the default setting is the highest power.
3) Add Water
Fill a glass measuring cup or glass bowl with 1 cup of water. Place both the cup of water and the dish in the microwave. For larger dishes and containers, place the cup of water on or inside the dish. Don't pour the water into the dish in question.
4) Set the Time
Microwave the dish and cup for one minute. If the dish or container is warm or hot after heating, the dish or container isn't microwave safe. If the dish or container is cool and the cup of water is hot, the dish or container is microwave safe. Note: if you placed the cup of water on or in the dish, the area around the cup may be warm. The rest of the dish should be cool if the dish is microwave safe.
Container Microwave Warnings and Tips:
Once you've established whether a container is microwave safe or not, place a piece of masking tape or a small mark with permanent marker on the bottom of non-microwave safe dishes. This will give you a quick way to determine the difference in the future.
Glass and glass ceramic cookware is microwave safe as long as it doesn't have gold or silver rims. Glass cups may or may not be microwave safe.
Never reuse frozen food trays and containers. These are made for one-time use only.
Avoid microwaving cold food-containers, such as butter tubs and whipped topping bowls. These can release chemicals into food when exposed to high heat.