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There is no command that can be issued to the BRTU to show you if there is an external clock present on E3. However, there are some visual tests and some monitor tests that can be run to give some clues. These two tests are not 100% precise, but should give you some kind of indication.  You can do either or both of the below tests:

 

1.        Check the CLK led on the front of the power supply (red extractor handles).  If it is lit, there is a clock attached to E3 on the back plane.  However, this test only shows the presence of a signal; it does not verify the correct speed and type of the clocking present.   The BITS clock input is located on the E3 connector on the BRTU back plane. The input is 1.544-Mbps AMI encoded, all ones signal from a stratum supply for the 6711-51 and 6711-52 Power Supply cards.  The correct input for the 6711-53 and 6711-54 Power Supply cards is 2.048 Mbps.  Only two contacts of the E3 six-pin connector are used for the BITS clock.

 

2.       In REACT Bring up a monitor access on any T1. Start a Monitor Parameter (MON-HCDS) test and watch the “Rate” field.   The following is based on experience and should not be construed to be a precise guide. My experience shows this:

 

+- 5   - there is a T1 bits clock wired to E3.

+-500 - there is nothing wired to E3

< 1,000 -there is a DDS 64k clock wired to E3