You can't tell whether it's multiples from the test stick. The line just means you are good and pregnant! I had the faintest line ever with DS and the DARKEST line ever with DD. I also had early ultrasounds so there's no chance of a vanishing twin. Similarly, you could have a faint positive test and be carrying twins. I think it's more to do with how long it's been since implantation and how productive the corpus luteum is than anything else.