How to know when the resault of subtraction is incorrect in a 4 bit binary subtraction circuit?
In general, you observe the carry into the most-significant bit and the carry out of the most-significant bit. If you try a few values you will observe a pattern in those two values when the result of the subtraction is incorrect.

If you don't have access to the carries then you will need to brute-force compare the magnitudes of the numbers. When computing \$A-B\$ the result will be incorrect if \$B &gt; A\$