The Long Answer:
"What is the purpose behind design?" This will at times get into the philosophical implications of design, which is beyond the science of ID. However, ID would provide a fruitful avenue of research to determine what underlying reasons there may be for why we see what we do in life. We know that intelligent agents often use various methods to achieve a particular goal - things shouldn't necessarily be taken for their face value as there may be overall goals that are being sought when what we are investigating is considered with other items that may be working in concert. In other words, intelligent agents often use many different things together to achieve a goal - these different items and their relation to each other cannot be understood without considering a "layering" of design for what is being investigated. We can use engineering design principles to help guide our study of things in nature which may allow us insight into areas that otherwise are a dead end from a purely naturalistic perspective.
We detect design by finding the tell-tale signs of design. Usually this is through seeking specified complexity. The specification corresponds to an unlikely pattern--usually we recognize a pattern by understanding its purpose, or have some notion of why a designer might cause it to exist. But in some cases, we might not fully understand the purpose of an object, yet still recognize it as designed. For example, the Smithsonian has presented intelligently-designed objects whose function, or intended function, is unknown to us. SETI operates under the presumption that it can recognize an intelligently designed signal from outer space even if it does not know exactly what the message is communicating. Many intelligent design theorists have hypothesized that non-coding DNA (popularly called 'junk-DNA') is designed, and has a function in the cell just waiting to be discovered.
So, the answer to this question is "sometimes we can tell the purpose of a designed object," depending on whether or not the specification is linked to a known function, or to some other recognizable intelligently designed pattern. This presents an active area of research for intelligent design theorists.
However, this issue is one of debate in intelligent design. There may be a variety of ways to recognized a designed object--and some of those ways might allow us to detect design without knowing purpose, and some of them might not. In biology, purpose is easy to detect because we can usually identify the purpose of an object in the manner in which it aids an organism.
Philosophical / Religious / Theological FAQs: