Knit vs Crochet
When those who do not participate in either craft see someone working with yarn, they always assume that the person is knitting. But in reality, this is not always the case. If you are not sure, there are a few different things you can look at to figure out which craft they are working with.
Tool
The first thing to look at is what type of tool they are crafting with. Knitting uses 2 needles and crochet uses only one. But looks can be deceiving as depending on the project they are working on, a knitter could be using a circular needle which is technically a double ended needle, so it is one piece but with the points on both ends of it. Crochet also has a version like this for a special type of crochet – Tunisian crochet.
Yarn
Typically, the type of yarn the person is working with is not going to give you any type of indication as to what they are working on. If you get up close though, you can tell from the way the yarn is working up. If it looks like the knit sweaters you buy, then it is a knitted item as machines can easily do that stitch. If it does not look like a sweater, than it is likely not knitting but crochet. Crochet is a more complicated stitch and is more difficult for machines to replicate.
Want to know more?
If you really want to know what the person is working on, go talk to them. Most crafters I have met are friendly and are always willing to share their craft with others. If you approach and tell them that their project looks wonderful, they are likely to be even more receptive and will answer any questions you may have about what they are working on.
Word of Caution
Never assume that the person is doing knitting or crocheting unless you can specifically recognize it. If you are not sure, it is best to ask which it is. Some crafters get very offended if they are doing one craft and you assume that they are doing the other. Most people will assume that a person is knitting as that is a popular term and many people associate a yarn with knitting but I know that some people who crochet get very defensive if you think they are knitting.
Regardless of whether a person is knitting or crocheting, if you see them out in public they are likely going to be safe to approach and talk to. If you are interested in starting either craft, they can likely direct you to a local craft store which offers classes.