The fake saffron is commonly known as American Saffron or Mexican Saffron. True saffron is Greek Saffron, Spanish Saffron or true Saffron.
As you will see from the photos below, fake saffron (top) is much lighter in colour than true saffron (bottom).
The fake saffron strands are straighter and coarser. True saffron strands are finer and usually have trumpet like shape ends.
True saffron is usually graded to ISO standards and any good manufacturer or supplier should provide this information on the labeling.