It can happen to the best of us, either because of forgetfulness or because piwik somehow reordered the contents of config.ini.php (I'm not 100% sure it is possible, but it seemed to happen to me recently, and I wrongly assumed that the line I had added to the top of the file had been simply removed by piwik... leading to weeks of lost statistics).
If this is present in the configuration:
[Tracker] record_statistics = 0
...simply tell the user. There are a couple of ways to do so, which are not mutually exclusive:
In the web UI, not only could we show the warning message, but it could also contain a button tracking, if such a thing is easy and safe to do.
I believe it is enough to show the warning message in the admin menu (and only for the superadmin, since he is the only one who likely has access to the config).
In 84a2e9e92e909fa7d60abfde6c78e4f79ded4c6b: Warning is now shown on General Settings page to warn user
when tracking is disabled via record_statistics = 0.
In 8ac1bc28ddb67c441f8088a3c22b1e8a38d81493: Refs #3551 Display message in all admin controlllers