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We recommend Qualys SSL Server Test which tests this and and a lot of other useful things (most of them related to you server configuration, not the certificates as such). For the chain specifically, look at the "Chain issues" heading where you want to see "None" (if you have trimmed the unnecessary certificates from the chain) or "Contains anchor" (if you have kept the full set).

Where can we check if there is a scheduled or emergency maintenance at this time?

You can check at https://sectigo.status.io/ and there is also a Subscribe button there that you can use to get updates via email, webhook or RSS.