Inboxbooster is a free tool that will reliably tell you when your mails are in spam.
When we detect an email going to spam on Gmail, I will personally email you (my address is firstname.lastname@example.org) to warn you about this issue.
Google spam filter can send the same email to the spam folder or to the inbox depending on the recipient. It is smart enough to detect I love receiving cold emails about food delivery but you hate them. In other cases, it will send all of your emails in spam. The first case is normal and expected behavior; the second means your day of prospecting will be affected and you’ll lose valuable leads on the way.
We only detect the latter; i.e. when your deliverability is in trouble and you need to fix it.