Many people think that the characteristic mark of a spider bite is two symmetrical marks in the middle of a bite. Sort of like this (Note: do not search for “spider bite” under Google Images if you are easily grossed out):
Well, I’m here to set the record straight. Yes, spiders do have fangs at the tips of their chelicera that they use to bite and inject venom into their prey (See: How Stuff Works – Spider Venom). Therefore, it is possible that if a human received a spider bite, there would be these distinctive fang marks. However, the reality is that spiders rarely bite humans at all, and if one did bite, most spiders are likely too small to ever leave two fang marks that are clearly visible to the human eye. In addition, two side-by-side marks are not always indicative of a spider bite. Many arthropods can bite more than once, leaving marks that might look like two fang marks to us, but really are two separate bites. Because of this, spiders can certainly be wrongly-accused of biting humans much more than they actually do (on the other hand, if the spider is small enough, it could get away with it!)
The easiest way to tell if a bite is from a spider or not is to catch the perpetrator in the act. Unfortunately, once you notice a bite, it is usually too late. If you are really worried that you’ve been bitten by one of the few dangerous spiders out there, then there will be other symptoms to look for. The number of bites that you see may hint at whether it is a spider bite or not, but is not a form of diagnosis, alone.