While the topic has been talked about obliquely, it seems time we came out and said it directly. How, exactly, do you know if you have found a jumping spider or not? The easiest way it to simply look at its eye arrangement. If it is a jumping spider, it will have two very large eyes in the center of its face, flanked by another slightly small pair. The picture below is an excellent example of a male Phidippus audax.
DISCLAIMER: These rules are guidelines only! Like much of science, there will be exceptions to rules such as these, no matter how carefully crafted. Please also keep in mind that these rules, while very general, are also focused mainly on the salticids of North America. The tropics will have exceptions to most, if not all, of these rules.
Rule 1: Where was the spider found?
a) On the ground - Maybe a salticid, see rule 2
b) On a vertical surface like a wall - Maybe a salticid, see rule 3
c) In a web - probably not a salticid, but see rule 4
Rule 2: When you walk up to it, how does it move?
a) Jumps! - probably a salticid
b) Runs away very fast - most likely not a salticid
Rule 3: If you wave your hand near/over it, what does it do?
a) Turns to face your hand, or where your hand came from - Salticid!
b) Runs like heck! - probably not a salticid
Rule 4: What is the spider doing in the web?
a) Building the web - NOT a salticid
b) Feeding on another spider - Maybe a salticid, see rule 5.
Rule 5: What do the legs look like?
a) Very long and spindly - Not a jumper
b) Proportional to the body size - probably a salticid
Rule 6: If capture and put in a container (NOT airtight!) for a day or two, what does it do?
a) Builds a tiny silken sleeping bag - Salticid!
b) Builds a traditional web - not a salticid
c) Stays on the bottom of the container and cannot seem to climb the walls - not a salticid
If you think you have found a jumping spider, we encourage you to Google the animal's description. As jumping spiders are so very colorful, and often uniquely patterned, you can often find the specimen your looking for without too much trouble. One great resource for this type of searching is Bugguide.net.
Just remember, that when in doubt, always err on the side of caution and contact an expert to help you identify the spider.