Quote from the LooseScrews.com.
"An easy way to tell if you have a freewheel is to look at the spacers between the cogs. If the spacers get bigger in diameter between the bigger cogs, you are looking at a freewheel....... If all of the spacers have the same diameter, you are looking at a cassette......."
Suntour made 6 and 7 speed cassettes.
Simply for informational purposes, Shimano made 5 and 6 speed cassettes.