If you feel less than sanguine about it and do a bit of the research to assuage your doubts, be prepared to feel even less confident afterwards. After years of casual research, the results I found have led me to file a lawsuit requesting access to the records  needed to perform an audit myself.
My statistical analysis shows patterns indicative of vote manipulation in machines. The manipulation is relatively small, compared with the inherent variability of election results, but it is consistent. These results form a pattern that goes across the nation and back a number of election cycles. I’ve downloaded data and verified the results from several states for myself. Furthermore, the manipulation is not limited to a single powerful operator. My assessment is that the data reveals multiple (at least two) agents working independently to successfully alter voting results.
What convinced me that vote fraud is possible?
For me, it started with the 2004 Ohio presidential election. In 2005 I obtained and examined that data and it confirmed what other statisticians had said – that the results were highly suspicious. The official report from the congressional hearing  on that election describes it as ‘the abuse and manipulation of electronic voting machines and the arbitrary and illegal behavior of a number of elected and election officials which effectively disenfranchised tens of thousands of voters in order to change the outcome of an election.’
For a thorough assessment, I recommend reading ‘Post-Election Audits: Restoring Trust in Elections ‘ published by the Brennan Center for Justice which includes a lengthy appendix of various well documented voting equipment problems. The audit in my county (Sedgwick, Kansas) that I wish to perform is similar to those recommended in this report. Currently, my county does not perform any sort of post-election verification of voting machine results, not even for recounts. I know because I requested such a recount after the November 2014 election. That request was denied.
The voting machine software used is proprietary and even the election officials are not allowed to inspect it. This is termed Black Box Voting  and combined with Direct Recording Electronic (DRE) voting, which permits touchscreen machines and does not require a paper trail allows a situation ripe for exploitation. In addition, as Harper’s Magazine reported in 2012 , the security of these machines is so lax that:
‘As recently as September 2011, a team at the U.S. Department of Energy’s Argonne National Laboratory hacked into one of Diebold’s old Accuvote touchscreen systems. Their report asserted that anyone with $26 in parts and an eighth-grade science education would be able to manipulate the outcome of an election….Johnston’s group also breached a system made by another industry giant, Sequoia, using the same “man in the middle” hack – a tiny wireless component that is inserted between the display screen and the main circuit board – which requires no knowledge of the actual voting software.’
Also, there’s the fact that the polls are red-shifted  (where there is systematic biasing toward Republican candidates) and have been for several election cycles.