In order to check if 301 redirect is complete in a site after migration or move then login to Google Search Console and select the site you are migrating from, Go to settings, click Open Report in crawl stats, you will see crawl requests breakdown.
Moved Permanently (301) should be at 100%.
This approach is only applicable if you are migrating the entire site and if the site you are migrating from was submitted to Google Search Console. If not so, then you have to give alteast 12 months to be sure that the 301 redirect is complete