The simplest way to check whether your contacts have expired are checking the supplier box in which the lenses were packaged. Typically, the outside of the box should have your prescription and expiration date of the lenses printed on it. Or you can know that based on determine the type of contact lenses you utilize, such as daily, monthly, quarterly lenses. For example, if you have daily lenses, you should replace a new pair next day. Another good expiration indicator is dryness when wearing your lenses. They are no longer of any use if you constantly blinking to moisten the contact lenses.
You should better set the time since you start to use the contact lenses after opening the packing. However, if you don't do this, you can guess the general time. When the contact lenses approach the expired time, the water content ability may decrease. And the contact lenses may not be clearly seen as it was at the start. You may feel all these symptoms by the feeling of your eyes. I suggest you not to use the expired contact lenses which may let your eyes get infection. In addition, the eyes may lack of oxygen because the oxygen permeability of contact lenses will decrease with the approaching expired time for the contact lenses.
Well, that is an easy question, if the contacts you have are not given by your friend or someone, then perhaps you know the duration of the contact lenses right? If you cannot remember , just bring the invoice to the clerk to see if they are expired or not. Basically, a pair of contacts will be good to use for a couple of months. Also, you should take care of them and fill solution with them to keep them clean.