You can use the octal dump, od, utility to display non-printable characters. Amongst other things, we have used it to detect the header bytes in a stream of binary packets.
Here's are some examples.
Displays in character format
# od -c a.out
Capture a serial stream and display in character format
# tip -9600 /dev/term/a | od -c
Capture a serial stream and display in hexadecimal format
# tip -9600 /dev/term/a | od -x
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