In general, with periodic sampling in an oscilloscope, if the frequency of the displayed sine wave changes when you change the time base (sec/division), then the displayed wave is an alias frequency.
Years ago, the first good digital oscilloscope in our lab at work was a decent LeCroy with CRT display and a deep memory (for that time) of 10k points. We were troubleshooting a large system and seemed to find a strong AC magnetic field at close to (but not equal to) 60 Hz, using a search coil. We looked all over the lab for the source, but when we changed the time base and saw the frequency change, we knew it was an alias. Speeding up the time base, we found that the signal displayed was actually from a 10 MHz system clock: the apparent 60 Hz (very clean sine wave) was due to the approximately 6 ppm difference between the internal clock of the LeCroy and the actual offending clock generator.