Checking the expiry date of your lip serum is crucial to ensure its effectiveness and safety. Here are the steps to do so:

1. Packaging: Look for the expiry date on the packaging of the product. It's usually printed on the bottom or the side of the box.

2. PAO Symbol: If you can't find a specific date, look for the Period After Opening (PAO) symbol, which is an open jar icon. This symbol indicates how many months the product is good for once it has been opened. For example, '12M' means the product is good for 12 months after opening.

3. Batch Code: If there's no expiry date or PAO symbol, you can check the batch code of the product. It's usually a combination of numbers and letters printed on the bottom of the product or on the crimp (the part of the tube where it's sealed). You can enter this code on cosmetic batch code checkers available online to find out the production date and estimate the expiry date.

4. Sensory Check: If all else fails, use your senses. If the lip serum has changed in color, texture, or smell, it's likely expired and should not be used.

Remember, it's always better to be safe than sorry. If you're unsure about the expiry date, it's best not to use the product.