Expired makeup can have a distinct, often unpleasant smell that can help you identify if it's past its prime. The exact scent can vary depending on the type of makeup product, but here are some general guidelines:

1. **Rancid or Sour Smell**: Lipsticks, lip glosses, or any other lip products can develop a rancid or sour smell when they expire. This is due to the degradation of oils and waxes in the product.

2. **Metallic or Chemical Smell**: Powder-based products like eyeshadows, blushes, and compact powders can develop a metallic or chemical smell. This could be due to the oxidation of the pigments and other ingredients.

3. **Mildew or Musty Smell**: Cream-based products like foundation, concealer, or cream blushes can develop a mildew or musty smell. This is often due to the growth of bacteria or fungi in the product.

4. **Loss of Fragrance**: Many makeup products are formulated with fragrances to make them more appealing. If your makeup no longer has its original scent or the fragrance has significantly diminished, it could be a sign that the product has expired.

Remember, your nose is not the only tool to identify expired makeup. Changes in color, texture, or separation of components are also signs that your makeup may be past its use-by date. Always prioritize your skin health over the lifespan of a product. If in doubt, throw it out.