There is no way to know if a campaign email landed in the spam folder or the inbox. In order to confirm that status, the recipient's account has to be checked.
Keeping in mind the security and privacy of the recipient's address, and for technical reasons, there can't be a system that can accurately tell which email landed in the lead's inbox and which landed in their spam folder.
There are certain tools like Glockapps, Mail Tester and Mailercheck in order to get a detailed delivery report. They provide a list of their own leads to which you can send emails to. Based on how many of those emails land in their inboxes, they generate a delivery report.
So in a nutshell, ultimately deliverability of the email accounts can only be speculated based on the following factors:-
Your own domain's reputation and how old it is
Reputation of email service provider's IP
Whether or not you are using SPF, DKIM AND DMARC
If there are any links/images that you have included - even the open tracking pixels can have an impact.
Testing tools like Mailercheck can give you a little sneak peek of how well your accounts or email content is performing. But, none of those tools can exactly tell which email of yours landed into the lead's inbox.
In Instantly, you can check your deliverability score (and if everything is set up correctly) by going to your sequence editor, clicking on 'preview email', then on 'check deliverability score'.
In a nutshell, there is a whole variety of factors that can affect deliverability. Also, could you please try this tool: https://www.mail-tester.com/ to see if your technical setup is done correctly?
If you want to run extensive deliverability tests, you can also run a Glock Apps test for your email accounts - https://glockapps.com/.
And, of course:
Be sure to keep your email content relevant and valuable to your recipients.
Optimize your subject lines: Use subject lines that accurately reflect the content of your email and avoid using misleading or clickbait subject lines.
Make it easy to unsubscribe: Provide a clear and easy-to-find unsubscribe link in your emails to help reduce the number of spam complaints, or let the user know they can unsubscribe any time by replying to you.
Regularly clean your email list: Remove inactive or invalid email addresses from your list to avoid bounces.
Test and optimize your email campaigns: Conduct A/B testing and analyze the results to continually improve the effectiveness of your email campaigns.
Use a reliable email service provider: Choose an email service provider that has a good reputation and provides the necessary tools and features to ensure deliverability.