Ultimately, Bob is right; you need to research each product individually. That said, most developers are used "one-shot," meaning that you use it and then immediately discard what you've used. The trick is that you can use most developers at multiple dilutions, and acceptable dilutions vary greatly from one product to another. For instance, D-76 is generally used at somewhere between stock strength (1 part developer plus 0 parts water, or 1+0 or 1:0 for short) and 1+3, whereas Rodinal is typically used at between 1+25 and 1+200 dilutions. Changing the dilution changes developing times and various characteristics of the finished negatives. Many developers can be re-used, but this is typically recommended only for stock strength (at least for developers like D-76 that are used at low dilutions) and it's usually done by large photofinishers; hobbyists typically dilute the developer and use it one-shot.
Some other types of chemistry are typically re-used. Stop bath can usually be re-used, and some varieties include an indicator dye that changes color from yellow to blue when the stop bath is exhausted. Fixer is almost always re-used a number of times, although some people prefer to dilute it down for one-shot use. Capacity is usually similar from one fixer to another, but it's not always identical. Hypo clear and wetting agents are generally used one-shot, and they're cheap enough that there's little reason to even consider re-using them.
