Though it isn’t going to be clear immediately how mobilegeddon is going to impact your overall traffic, I have been getting a lot of questions about how to check mobile rankings. This isn’t as easily done as one might think, especially if you have a mobile responsive site rather than a separate mobile version. One way to keep an eye on it is through the “Search Queries” under “Search Traffic” in Google Webmaster Tools (see image). If you set the filter on “Mobile”, you can look at your impressions and clicks over time and notice a sharp change if you look at data over time. Day to day searches can indeed just have naturally sharp changes, but if you see a big drop or jump relative to larger periods of time, you are probably seeing Mobilegeddon. For example, if impression are in the hundreds most days with some dips of 25%-50% here and there but it suddenly drops to 10, you likely have an issue. Or if it suddenly jumps 100% you may be experiencing a benefit from being mobile friendly.
Just another reason to make sure you have Google Webmaster Tools set up for your site. They would have sent you a warning prior telling you if you had mobile friendliness issues. They also give you some extra data that isn’t as quickly seen in analytics.