If it is a logging only proxy, you cannot. As you speak of Wifi, I assume that you connect through DHCP, so the default router is obtained through DHCP. That router actually proxies all traffic between its Wifi interface and its internet interface. From the outside, you cannot guess whether a spying system has been installed here.
On the fly rewriting of packets at this low level is rather complex and is uncommon: those are generally only filtering proxies that can block unallowed traffic. You could imagine one specific tool that for example consistently adds a new address on the mail envelop (not the
To: header, but the
RCPT TO: SMTP command). But as it is simpler to just log all traffic on selected ports, I have neved heard of such a tool.
TL/DR: you cannot detect a proxy when you connect to a Wifi network that you do not own, because by definition the Wifi hotspot is a router and could log all the traffic that goes through it. So you should assume that everything that passes in plain text can be spied.
That is for the low level proxying possibility. A more common usage is that ISP block SMTP traffic (port 25) that is not for their own server. This is a best practice to avoid open mail relays. But the secure ports using SSL/TLS or requiring authentication (465 and 587) are generally not blocked.