To determine if lactic acid is expired, you can consider the following factors:

1. Check the expiration date: Look for the expiration date on the packaging or label of the product. If the date has passed, it is likely that the lactic acid has expired.

2. Observe changes in appearance: Over time, lactic acid may undergo changes in color, consistency, or texture. If you notice any significant changes, such as a change in color from clear to cloudy or the formation of sediment, it may indicate that the lactic acid has expired.

3. Smell test: Lactic acid typically has a slightly sour or acidic odor. However, if you notice a strong, unpleasant, or rancid smell, it could be an indication that the lactic acid has expired.

4. Perform a patch test: If you are unsure about the expiration status of the lactic acid, you can perform a patch test on a small area of your skin. Apply a small amount of the product and wait for 24-48 hours to observe any adverse reactions. If you experience any redness, irritation, or discomfort, it is best to discard the product as it may have expired.

It is important to note that using expired skincare products can potentially cause skin irritation, decreased efficacy, or even bacterial contamination. Therefore, it is always recommended to check the expiration date and discard any expired products.