You check hashes (from the web site) after you copy the files to media you trust, and then any tampering is really interesting . . .
You can find well-known hashes to check your install media with online (assuming you can trust the media because it comes wrapped up in a nice package is a bit naive...). What you cannot check however, is a USB key firmware (ring-0?) malware, that lives completely outside of disk space, and thus is beyond reach, but might have limited (or even complete) access to the system it is plugged to. CDs and DVDs (don't know, but perhaps SD/compact flash/etc. cards too?) are thus the last mainstream media that come with 'no firmware attached'.
> What happens when a malicious actor leaves physical evidence all over?