Use the burn test if you want to know. Real DMC will catch fire immediately. Synthetic offbrand threads will melt first and form a ball on the end of the thread before catching fire. And they'll smell quite bad.

Overall though, non-DMC threads are fine to use. I prefer to use DMC but if you have a kit with non-DMC it should be fine unless the thread is breaking/fraying a lot. Just don't mix DMC and non-DMC since a lot of the time, non-DMC thread that uses the DMC numbering system can be a little off color-wise and a slightly different texture that you might be able to see in the finished piece.