See how the tea is packaged and stored. Pu-erh should generally be stored and aged in a cool, dry area, away from other aromas. It is usually stored in its original paper wrapping.
Smell the tea. Good pu-erh tea should smell clear and distinctly "tea-ish". Depending on the age, you may detect smoky or woody aroma. 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. Ancient raw pu-erh tea cakes should look reddish. Younger pu-erh 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 businesses can allow one to taste teas of different characters at different ages, enabling people to distinguish what's good.
Familiarize yourself with pricing online before purchasing, even if you intend to buy in a local tea store. Pu-erh is made by different factories, in specific batches. Major factories are Menghai, Xiaguan, and Shuangjiang Mengku, and there are many others. Some tea shops that do not specialize in Pu-erh would sell low-quality Pu-erh tea at high prices.
Have a clear idea of what you want. By being able to specify your target or type of Pu-erh tea (e.g. Shou, Sheng, aged, new) you will be able to focus on the teas you are looking for and spend more time sampling and evaluating teas that you are interested in. (Although for beginners this is not necessary and ideally you should be trying as many types of teas as you can to know what you like and want to pursue in the future.)
When you're tasting tea, don't listen to what anyone else tells you - even if there are 10 people sitting around you saying how good a tea is, if you don't have the same experience, then stick with your own view. Experiential learning is everything.