Can anyone tell me just what I should be seeing when I look at pattern/EAPG glass with a black light!?

I have been asked to look at some old pattern glass that I am trying to sell with a black light, and tell the potential customer what I see. What would I see? Does this apply to ALL old pattern glass?? Should I invest in a black light for future purchases? Do I need this to identify all old pattern glass/EAPG glass? Thank you!