If you wear makeup, one of the best tips I can give you is to put a little label on each item with the expiration date. This will help you keep track of what's in your collection, make sure everything is being used up, and will help you know when to toss out expired items. No makeup product, regardless of the price tag, is worth a trip to the eye doctor because you used a product past it's prime and developed an infection. ;)
Some makeup comes with a tiny box and a number beside it on the packaging; this tells you to use it within that amount of time before it expires. (Example: 6M = once opened, you need to use your product within 6 months, then replace it.) Not all products have that information because in the US, we don't have national standards for cosmetics. If you're not certain when your item expires, you can use a website like www.Cosmetic.net to enter the brand name, and the lot code on the product. While they don't have every brand on their website, they do have a large variety, so hopefully it will be helpful. (The lot code can be tiny, and often branded into the product itself, or on the label.) That website can tell you when the product was made (what batch it was in, the year, etc) and when it expires for many products. Just as a good rule of thumb, many products, even ones you haven't opened yet, have a shelf life of about 3 years before the chemicals begin to destabilize, which can make the products less effective. This means if you have products that have say in you're collection unopened/unused for several years, it may still be bad because it's reached its shelf life. Natural/organic cosmetics expire more quickly because of the ingredients they use in them. This is why it's important to label everything as soon as you purchase it, and know when your products need to be replaced.
I hope this little tip helps you enjoy your makeup more safely!
(I used a label maker to put little date labels on each of my items so I'll know when they expire. The picture I've included is just an example of a few ways you can adhere to the labels to your products. If you do not have a label maker, you could also use a fine tipped sharpie and place a piece of tape over it to prevent smudging. I also recommend apps like BeautyCache for photographing each item, listing the price, brand, shade, and dates so you know what's in your collection that way as well. If your on a beauty low/no-buy, that app is an excellent way to keep you on track!