How would we (realistically) know if we get a spoofed email

I always wanted to know if it was possible to actually do this type of thing, how could we realistically know if we are being spoofed.

If you look at the from there, you’ll see something like "from zaid@fbi.com vial some.suspicious.server.com so that’s a big indication, you an also look at the source of the message and you’ll see that its sent using a suspicious server.