Oriental rugs are valued as collectible items across the world. It isn't hard to share with a handmade rug from a machine made one, but beyond that, value is harder to distinguish.
A handmade rug has the same pattern on the back since it is wearing the front. A machine made rug will look like it features a mesh that partially obscures the design. So the very first thing you look at when making an option, could be the back. If it's handmade, then it's worth something. If it's not, then it isn't going to retain value.
Beyond that, the value of a handmade rug depends upon several factors:
Knot count. This really is among the easier things to define, because it can be defined in absolutes. You could block out a square inch and do some counting and multiplication
rugs to get a fairly great idea of the knots per square inch. They vary between 20 and 500 knots per square inch. Higher generally presumes higher value, everything else being equal.
Condition. Another easy to spot factor. Wear, fading, fraying, or stains will decrease the value. The amount to which this occurs depends upon other factors, but major damage may have a significant influence on value.
Origin. Rugs go in and out of fashion. Even rugs that are out of fashion can be quite valuable, but those who are trendy could be more prized, and therefore command higher prices. What is out, comes in though. Currently, tribal rugs would be the trendy collector's items, but city made rugs which are larger, with higher knot counts, can nevertheless be very valuable and desirable.
Materials. Rugs may be created from a number of fibers, wool being the absolute most common. Design. Some designs are more prized than others. Such as the origin though designs go in and out of fashion, but always retain value.
Age. Another major factor, and one which can be harder to judge than others. Age is judged by condition, colors of the fibers, finishing styles, patterns, and probable origin. Age of a carpet can never be precisely pinpointed without documentation (rare in a true antique), but a professional can judge fairly accurately inside a decade or so.
You can't judge all these factors without some professional training or experience. But you can learn the fundamentals, and have a good idea of whether or not someone is trying to scam you by presenting something as besides what it is.
Value originates from the masterful work and quality put into it in the very first place. Without those hours of personal time put in selection and preparation of the materials, and then to the creation of the job, it does not have any lasting value. This is what separates a handmade rug from a machine made rug, even when they look similar.
Value in a handmade rug has a tendency to grow as time passes, notwithstanding market fluctuations. It is just a relatively "safe" investment for many who wish to invest in the arts. Properly looked after and preserved, they hold their value well.