Up in the sky, is that a drone, a plane or a helicopter?Experts who study unmanned aircraft systems — better known as drones — say it can be tough to tell from miles away. But there are clues.A light in the sky at night can easily be misinterpreted, according to John Slaughter, director of the Unmanned Aircraft Systems Research and Operations Center at the University of Maryland."You can't just walk outside and say, 'Oh, that's not a drone,' or 'That is one.' All you can really factually say is 'I saw a light in the sky,'" Slaughter said.Dozens of mysterious nighttime flights first spotted in New Jersey last month and now being seen across the U.S. have raised concerns among residents and officials. Part of the worry stems from the flying objects initially being spotted near the Picatinny Arsenal, a U.S. military research and manufacturing facility, and over President-elect Donald Trump's golf course in Bedminster. Planes and helicopters all have a flashing lights and typically at least one red anti-collision light, often two. They also sport navigation lights, which are steady, red and green colored lights like the ones found on boats. Many will also have flashing white strobe lights at their wingtips. And they have bright landing lights. Drones flying at night are only required to have one bright, anti-collision light that's visible 3 miles (5 kilometers) away. But drone owners can add other lights, so some have more than one. Drones are smaller, so when they have multiple lights they are close together, but at night it's often not possible to figure out precisely how far away they are."A light is just a bright point," Slaughter said. "And it might be 100 yards (90 meters) away, it could be literally 40 miles (65 kilometers) away and it looks the same." Drones tend to be quieter and make a more high-pitched buzzing noise than what's generated by the jet engines or propellers that drive planes and helicopters. But bigger drones can be louder, and the sound may be difficult to discern from a distance.Drones heavier than about a half-pound (0.2 kilograms) may have an identifying number displayed on the outside."You certainly wouldn't see it at night, and you'd have to be up close to see it, you know, in the daytime," said Paul R. Snyder, director of the Unmanned Aircraft System program in the University of North Dakota's Aerospace Sciences School. Planes and even helicopters tend to move smoothly in the air, but multi-copter drones can stop on a dime, pivot 90 degrees and reverse course, Slaughter said."That kind of motion can give you a clue that you're looking at a drone, not an aircraft," he said.And the vast majority of drones will be operating below 400 feet (122 meters), following federal regulations.