You can check their SPF and DKIM records in the header of the email, or by using a tool like https://mxtoolbox.com. Spam reports do impact a sender’s reputation, yes, but good to “spam” actually counts as delivery in email (because the alternative is outright rejection, and no delivery). You can avoid email going to spam by adding the sender to your address book/contact list, which should skip the inbox sorting for those emails.