“How do I know how old my gran’s jewelery is?”
What type of jewelery is it? If it is made from a precious metal like gold or silver it will almost certainly have a hallmark. Depending on the maker of the jewelery their maker’s mark will often have changed many times throughout their history, and it is often possible to use this to date the jewelery. There are a number of websites and fairly inexpensive books which will help you to locate certain hallmarks and find out the production dates associated with such hallmarks.
Other factors, such as standard letters used around the turn of the century make dating hallmarks relatively easy. The style of the jewelery will often also give an indication as to the date is was designed, for example the length of a bracelet, or the use of certain precious stones as well as the overall aesthetic style of the piece.
The best advice I can give you without looking at your jewelery is to try and find out the history of the items from other family members, perhaps somebody knows who the item originally belonged to? Failing this, a number of experts with considerable knowledge of the antique jewelery market will be able to help you out.