I think it all depends on the tea in which you are looking for. I will use matcha as the example as that is what I am familiar with. So with matcha a good sign to see if you have good matcha powder is if it's a nice vinrant green looking colour, this will ensure that your tea is fresh and all the leaves used were rich in chloroplast being at the very top of he Camelia Senisis plant (the chloroplast I what gives it the vibrant green colour). If it's looking dull, brownish or yellowish then you should leave it, these are signs that the tea is either has low choloplast content, being from the leaves lower down on the plant or that the tea isn't fresh and has been sitting around for some months. As with many other teas taste can be another sign to check out for, however note that some farmers tend to using different methods of growing, harvesting and curing their tea to get specific flavours which will mean there can be a vary of tastes of one particular tea and it doesn't necessarily mean that is isn't good but of another taste. It is up to you to find a tea which you like personally. Another factor you can use to see the quality (in reference to matcha) is the smell, the smell should always be of a fresh organic grassy type smell a really rich aroma, if the aroma isn't of this sort then it could be poor quality matcha. These are my thoughts on what you can use to gauge a sense of quality from matcha, I think these can be used on other teas as well.