The state of tea can be made out by the aroma. A tea connoisseur will easily make out whether the brew is spoiled or fresh from the characteristic aroma of dry leaf but it will be difficult for an amateur to make out the rancidity in leaf in the early stages.
The other way is to pour tea infusion in the pot and inhale over it immediately if the rising aroma is pleasant sweet fruity or flowery...in short it smells nice the leaf is fresh and good. On the other hand a smoky burned aroma like that of everything stale means the tea is spoiled.
Sometimes stale leaves will exude no aroma at all this is a sure shot sign that you have purchased stale leaves which have been lying in store well past its storage period.
Different teas have different store life as green tea and white tea can withstand low period of storage where as black leaf can be stored for a considerable longer period. Therefore in early history black leaf was a preferred export to regions that where far off and where tea was not cultivated at all.
The aroma changes in every stage from freshly prepared drink to when it has slightly cooled and much after that. With practice one should be able to discern all the characteristic of tea leaf…the one which he regularly consumes.