Check for update button on Android does nothing expecting for manually checking for updates in case you haven’t got an update notification automatically. However, back in September manually pushing the check for button update on Nexus and Pixel phones would fetch an available update even if the background check wouldn’t.
However, an update to Google Play Services has broken the feature, and it was reported that the function would return only in 2018. Now since it is 2018, the feature is back again working on the said phones. The v12.2.09 beta that rolled out yesterday is said to have fixed the button, and it was successfully fetching the available updates when checked manually.
The feature’s return was spotted first spotted on Reddit, and since it is only limited to Pixel and Nexus series, it was working on first gen and second generation Pixel smartphones. You can test out the Google Play Services 12.2.09 by signing up for the beta on Play Store or manually download the APK and side-load it.
So after installing the latest APK, if you are in the January security update, you can try checking for February security patch which was rolled out earlier this week.