Ask the dealer where the rug came from. If he or she tells you that it was made in any place other than Iran, it is not a real Persian rug.
Measure the number of knots per square inch (KPSI) using your tape measure. Multiplying the number of knots on one side of a square inch on the front of the rug by the number of knots on a side perpendicular to the first. A quality Persian rug will have a much higher number than a machine-made rug, usually around 150 to 250, but this is only part of the way to determine if your rug is genuine and of high quality.
Look at the placement of knots on the top of the rug. Handmade rugs will have a strong design connection between every knot in the rug, there will be no single knots standing alone among other colors. Machine-made rugs do employ single knots as they shade from one color into another.
Fold the rug in half. If it flops more like a blanket than like a rigid sheet of paper, then it is more likely to be hand-made.
Look at the angles of the corners, and at the width of the knots in relation to one another. Unlike machine made rugs, handmade rugs will have corners that are not quite square, and the size of the knots will vary slightly.