There are two ways to view the currently customized alert settings in SQL Monitor.
Using the SQL Monitor UI
On the Configuration>Alert settings page, at the left of the screen, there is a hierarchy tree of all the entities. Any entity that is bold in this list indicates that there is an alert customized at this level and the number in parentheses after the entity name indicates the number of alerts customized at that specific level. This count does not include any at sub-levels. You may need to click the + symbol on the vertical line to expand the left panel.
In this image you can see there is one alert customized at the "All Servers" level, two alerts customized at the "Production" group level, one at the "ps-alexb2" machine level and one each at "sql2008r2" and "sql2012" instance levels:
When you select the entity level in the hierarchy tree, on the right-hand side you will then see the customized alerts are in bold font and have <This level> in the "Inherited from" column
Using the support log retrieval
A CSV containing a list of the customized alerts is provided from the support log retrieval function on the Configuration>About page by clicking on the "Retrieve all log files" link in the Diagnostics section at the bottom of the page.
This will produce a zip file that includes the AlertConfiguration.csv file with every customized alert and the details thereof listed. Inside of the zip file there will be a folder for each base monitor, the initial base monitor will be listed as localhost. Inside of the base monitor folder open the RepositoryInformation folder and then choose the aforementioned AlertConfiguration.csv file.
The file lists all of the customized alerts and also includes all alerts for custom metrics at the root level (in addition to any other customization). The first column indicates at what level of the hierarchy the alert is customized, the name of the entity is in columns 2-4, the alert type and name is in columns 5 and 6, with the sub-type for custom metrics that you can correlate to the the [settings].[CustomAlertDefinitions] table in column 7. The actual configuration XML is listed in column 8, whether it is enabled (0 or 1) is in column 9, whether any alert notification types are disabled (0 or 1) column 10 and the custom email addresses in column 11.