Q: How could you tell that the product doesn't suit your skin?
A: If your have the right skin care, it would help improve your complexion and you'll notice within a week to a month I'd say. You might not see a big or dramatic result, but you could tell that your complexion is getting better. For example, if you have acne prone skin, you'll breakout less. If you have dry skin, you'll feel less tight sensation.
If it's not, your skin would give an reaction, you might be breakout, having irritation, dryness or your complexion doesn't show an improvement. Like the most notice on the package 'Stop if irritation occurs'. There some people said that you should continue despite the ugly reaction. I said hell no. If you continue the usage, there's a chance that the reaction worsen, and ruin your skin.
To choose the right skincare you need to know you skin type really well, then you could decide what your skin needs.