How do you validate whether an indexation fix actually worked?

For a canonical fix, check:

• source HTML;
• rendered HTML if relevant;
• canonical;
• sitemap;
• internal links;
• URL Inspection;
• Google-selected canonical after reprocessing.

For a noindex implementation:

• response;
• robots meta/header;
• crawl accessibility;
• Search Console state over time.

For a migration:

• old URL;
• redirect;
• destination;
• canonical;
• sitemap;
• indexed destination.

Remember that Google reprocessing is not instantaneous. Canonical clusters can take time to be reevaluated.