I just stumbled about this question by trying to find out the same on v.2.2
Unfortunately the collection of a repository isn't public accessible from within objects using it. For me the best way - for now - was to use xdebug (https://xdebug.org/) and the corresponding watcher feature of my IDE (Intellj Ultimate => see https://www.jetbrains.com/idea/). In my case I had done some filtering on the cms page repository and wanted to see the query, when it is done.
I did this by adding a brakpoint in \Magento\Cms\Model\PageRepository::getList (161).
When the debugger stopped there, I was able to watch the query using the watcher by adding the following expression to the watchlist:
That is the known Magento 1 style.
I hope this helps a little bit to get further.
If someone wanted to know how to use xdebug with IntelliJ or PHPStorm, have a look at https://www.ask-sheldon.com/php-xdebug/.