I suspect it would have been too much trouble for you to give us the make and model of the case?
Now you have to do it the hard way.
1. You can either Google the case make/model and see if you can find the specifications fo rthe case and in that the motherboard form factor(s) it supports,
2. You can Google for "motherboard templates" and then from that you may want to use a site like http://www.formfactors.org/formfactor.asp
(there are easier sites, but I can't find it now) to determine the hole spacing for the ATX and micro-ATX form factors. Then you can measure your case backplate and see which of those have holes.