heya Tetelle - You can manually verify this...the only drawback of the methods below is that these are post deployment checks...
Check Search console
First place to look is search console...The Language section of the International Targeting page in search consoles shows a high level view on errors from hreflang links throughout your site. Here is a page with more details on checking that out: https://support.google.com/webmaster.../6061244?hl=en (this page also includes info on common issues with implementation) . I would wait two weeks or so to check this, Search console is kinda slow and your sitemap needs to be recached....
The second check to do is to run manual queries in each of your target languages. You mention above that in google.de that you would just see the .de version and all other would be deindexed, this is not necessarily correct from my experience , even with correct implementation. For the query that you search the .de version should appear first. If the site that you're working on is new, you can manually test this with brand and partial branded queries to make sure its showing up correctly.
Check referrals a couple weeks after deployment
The last way to check this in a scaled fashion is segment traffic by source / medium in Google Analytics [or whatever other analytics provider you're using] and check to see that incoming traffic is going to the proper presence through looking at the Geolocation data for referrers. If you're getting a lot of traffic from google from german IP's to a different language or geotarget versions of your site, that is an indicator that something may not be working as intended as well.... the only requirement here is you need to wait a couple weeks to be reindexed before you can run this check...
Hope some of this helps!
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