In our case it's quickly obvious since our server sends verification codes when people sign up for online accounts or change their passwords. We'll see a pattern of customer complaints from certain mail services within a day of being on any major blacklist.

Honestly, we usually find out after communication has failed and some other form of communication is used.

It's usually worthwhile to remind businesses that when they use "free" services like privacy-paid outlook.com and Gmail, they'll get what they pay for, and if their communications really matter, they should find proper email providers.

I can never know for sure if email is avoiding the spam folder, but everything tells me if it does happen, it's quite rare now.

First, I will send emails to test accounts I have set up with major email providers (gmail, yahoo, outlook, etc). I used to do this often, but they have all been going to the inbox on my last few tests, so I haven't done this recently.

Second, is feedback I get from recipients. When I send an email and ask for a reply, I'll get a reply. I also used to hear a few times a month, "sorry for the delay. I found your email in my spam folder!". This has gone away.