Then ask them to send you something like a series of letters and #'s from that .mil email account.One correction to that: don't ask them to email you something from that .mil address. Instead, send email to that address YOURSELF. If it exists, they will receive it. If it doesn't, it will just bounce, or some unrelated military guy will receive it and will be VERY surprised to get your message. By emailing them yourself you will avoid email address spoofing.
Actually, that was discussed here:
https://www.romancescam.com/forum/viewtopic.php?f=91&t=55756
This is what Lieutenant Colonel Gordon Hannett (ghannett) says:
A scammer can tell you his fake military e-mail address and he may even be able to create and send an e-mail that appears to be coming from an official military e-mail account. What he can't do is RECEIVE an e-mail going to an official military account (because he doesn't have such an account). So, if you hit "Compose" (not REPLY) and type in the e-mail address he's given you, your e-mail should go through if it's legitimate. If you're talking to a SCAMMER, one of two things will happen: it will be returned as undeliverable or it will go to a real soldier who has no connection to your SCAMMER. As an example, John Smith is probably the most common male name in America. I guarantee that there are dozens of American soldiers with that name. One of them is bound to have the e-mail address john.smith@us.army.mil or john.smith.mil@mail.mil. If your SCAMMER gives you one of those e-mail addresses, your e-mail will go to the REAL soldier with that name and e-mail address. Your SCAMMER will never see the e-mail.
Please pay close attention to what Lt.Col. Gordon Hannett says in that topic, he has been our scam adviser on RS for many years, helped a lot of victims. We so much appreciate his help.