If the upper quartile of one data set is below the lower quartile of the other data set, is there a significant difference between the two data sets?
Yes - there is a significant difference
If the upper quartile of one data set lies between the lower quartile and median of the other data set, is there a significant difference between the two data sets?
There is probably a significant difference
If the median of one data set lies between the median and either quartile of the other data set, is there a significant difference between the two data sets?
No - there is no significant difference