See how the tea is packaged and stored. Pu-erh should generally be stored and aged in a dark, cool, dry area, away from other aromas, where it has access to some airflow. It is usually stored in its original paper wrapping. Tea shops that have elaborate or airtight, sealed packaging for Pu-erh cakes often are not offering the best Pu-erh for the best deals and are often not storing it properly.
Smell the tea. Good pu-erh tea should smell clear and distinctly “tea-ish”. Depending on the age, you may detect smoky or woody aromas. The tea should not have odd odours, and it should not smell moldy. Tea absorbs smell rather easily; thus, if improperly kept, the tea can pick up funny smells from anything: cooking smells, spices, etc.
Notice the tea’s appearance. Very ancient green pu-erh tea cakes should look reddish. Newer pu-erhs will be more yellow-to-greenish, but they will never look pure black. The tea cake should not have white/yellow dots which might be mold or yeast forming. When buying expensive tea cakes, it’s generally preferred for the cakes to be whole, without large cracks. Sometimes, people might sample expensive tea cakes by using a knife to scrape the centre of the depression behind the tea cake, flaking a few leaves off. This is seldom noticed, but be aware that you’ve lost a bit of tea if your cake has been sampled.
Do a taste test. This can only come with experience and more exposure to pu-erh tea. Purchasing samples from pu-erh tea museums can allow one to taste teas of different ages, enabling people to distinguish what’s good.