You can't tell if your old sunglasses are safe by yourself. You could take it to any optical store and ask them to check it. The optical stores have a UV meter which is used to check whether your sunglasses have UV protection.
You can take your old sunglasses out under the sunshine. If they change the color, then they still have UV protection and are safe for your eyes. If they don't chnage color, then they might not have UV protection no longer. You can replace the lenses with new ones.
Dude, if you can't tell whether your sunglasses are safe or not, how about buy another pair of new sunglasses with UVA protection. There are so many online eyeglasses stores. the sunglasses with UV coatings are cheaper than they were used to be. And there are online customer services too. You can ask the customer service staff whether the sunglasses sold by them are UVA protection or not before you place an order. Good luck!
If you doubt about the quality of the sunglasses, you can ask the seller to prove it for you. I don't know whether your old sunglasses are safe or not because if there are some scratches on the lenses, the lenses can't block all the UV rays for you so your eyes will be damaged if you still use them in future.
If your old glasses can't protect your eyes from the UV rays anymore, you'd better throw them away and buy a pair of new sunglasses. You can take your sunglasses to a local eyeglasses store and ask the optician to check your sunglasses. If the optician told you that your sunglasses are in good condition, you can use them again.
I think the sunglasses can be used again and again as long as they can provide me clear vision. Besides, my sunglasses are prescription sunglasses and also with UVA protection. If I don't need to change the prescription, I don't need to change my sunglasses. I often take good care of my sunglasses. I will clean them and store them in case when I don't need to wear them. So now they are still in good condition. They works the same as they were used to be.
You can test your old sunglasses at home. You can borrow the UV flashlight and turn on it after turning off other lights at home, and shine the UV light on the lenses of your old sunglasses. If they are fluorescent, it proved that your old sunglasses are of UV protection. But this can't show you how much UV light your sunglasses can filter for you, while at least they are actually doing something for you.