The essential test to discover whether the US elections were rigged is this: to compare the differences between opinion and exit poll results and actual results in states that used electronic voting with those that didn't.

If there is found to be a statistically significant difference from the exit poll figures in states that used the suspect electronic voting machines, that would constitute strong evidence that the elections were rigged.

Apparently there is some such evidence - see for example:
http://www.newstarget.com/002076.html.