Hi,

It is sometimes very hard to tell, and sometimes it is impossible because people have crossed dione with bimaculata.
The best way you can see the differnce is the shape of the head.
Bimaculata have a very Narrow and long head and dione Broad and blunt.
Also the incubation time of eggs. Dione average about 20-30 and bimaculata 40 .

Here is a good article about it:
http://www.ratsnakefoundation.org/modules/article/view.article.php?c6/42

Best regards,
Gidi

ps You can post a pic of them here on kingsnake I am pretty sure some people can tell you what species it is. Good luck!