The only way that i think you can know for sure is to keep a day by day account of who is on your Gb and their donations, or message everyone on that level and ask if they did. other than that its just a guess. Although as others have mentioned you can narrow the list of suspects.
Im in the same boat, trying to figure who leveled my AO yesterday. Its not the end of the world, but if they on my friends list, they wont be for much longer.
There is another way of narrowing the list of suspects, check your events history, see what time the Gb was leveled then check if anyone on the Gb aided you or sat in your tavern a minute or 2 before or after that time..