A way to check is using Gmail.
Sometimes it's obvious:
But most aren't, so you can click on the 3 dots and select show original
Here, for example, you can see this other one was sent using Zoho Mail's SMTP
By the way, I was also seeing emails sometimes land in the spam box using SMTP. Once I setup SPF, DKIM, and DMARC for my domain, I've never seen them land in spam again.
There are three main types of authentication used
to prove to mailbox providers that your email is worth being placed in
SPF (Sender Policy Framework) is an IP address-based authentication method used to compare the sender’s IP indicated in the
domain’s DNS record with a list of IPs authorized to send from that
DKIM (Domain Keys Identified Mail) is a method for validating domains
through a publicly available cryptographic signature. This signature
is verified on the receiving end using the DNS record.
DMARC (Domain-based Message Authentication, Reporting, and Conformance) is a protocol aimed at reducing email phishing and
spoofing. It allows a sender to specify that their email is properly
authenticated using DKIM and SPF records.
Senders who authenticate their messages with all three methods may see
less filtering and fewer emails going to spam.
source: How to Prevent Emails from Going to Spam: The Ultimate Guide