In general, there are two main differences between singlemode and multimode connectors. The first and most obvious would be the color of the strain relief boot. A singlemode boot is generally blue or white, while multimode boots are normally black or beige.
But, the main difference is in the ferrule of the connector. Singlemode connectors will almost always have a zironia (ceramic) ferrule while multimode connectors can have stainless steel (Nickel-Silver), composite (plasitc), or zirconia. The size of the hole in the center of the ferrule will determine the correct fiber type that the connector is designed for. A standard singlemode zirconia ferrule will usually have a ferrule hole of about 126 microns, while standard multimode ferrules will be closer to 127-128 microns. The difference is very small, but can make a large impact on insertion loss if the larger multimode ferrule is used on singlemode fiber.
So, the best way to tell the two connectors apart?… Keep them in their original packaging until ready for use.