After spending an embarrassing amount of time searching on the internet how to read a simple DCD line of a serial port from a linux shell, I found a hint here.  Behold how to read status on all your serial ports…

watch -n 1 cat /proc/tty/driver/serial

root@vapn:/proc/tty/driver# cat /proc/tty/driver/serial
serinfo:1.0 driver revision:
0: uart:16550A port:000003F8 irq:4 tx:11 rx:0 CD
1: uart:16550A port:000002F8 irq:3 tx:0 rx:0 CTS
2: uart:unknown port:000003E8 irq:4
3: uart:unknown port:000002E8 irq:3
4: uart:unknown port:00000000 irq:0
5: uart:unknown port:00000000 irq:0
6: uart:unknown port:00000000 irq:0
7: uart:unknown port:00000000 irq:0
8: uart:unknown port:00000000 irq:0
9: uart:unknown port:00000000 irq:0
10: uart:unknown port:00000000 irq:0
11: uart:unknown port:00000000 irq:0
12: uart:unknown port:00000000 irq:0
13: uart:unknown port:00000000 irq:0
14: uart:unknown port:00000000 irq:0
15: uart:unknown port:00000000 irq:0
16: uart:unknown port:00000000 irq:0
17: uart:unknown port:00000000 irq:0
18: uart:unknown port:00000000 irq:0
19: uart:unknown port:00000000 irq:0
20: uart:unknown port:00000000 irq:0
21: uart:unknown port:00000000 irq:0
22: uart:unknown port:00000000 irq:0
23: uart:unknown port:00000000 irq:0
24: uart:unknown port:00000000 irq:0
25: uart:unknown port:00000000 irq:0
26: uart:unknown port:00000000 irq:0
27: uart:unknown port:00000000 irq:0
28: uart:unknown port:00000000 irq:0
29: uart:unknown port:00000000 irq:0
30: uart:unknown port:00000000 irq:0
31: uart:unknown port:00000000 irq:0

The CD (aka DCD) line was of specific interest to me concerning a WWVB clock input. As you can see above, the asserted status shows up just fine to help my ensure the WWVB signal is really making it into the OS for use by radioclkd2.

Wiring confirmed, onward to making WWVB work with NTP.

Sometimes the simplest tasks take the most time to learn.