Hi,
If the 7A really is the expectable peak current...then you need a low ohmic resistor to keep voltage drop and power dissipation low.
Let's say power dissipation shold be be about 1W at 7A:
P = I^2 × R --> R = P / I^2 = 1W / (7A× 7A) = about 20mOhms.
The voltage drop at 7A is I × R = about 140mVolts.
But to detect a low current motor, then you need to detect currents below 100mA.
To get some margin ... let's calculate with 50mA.
50mA × 20mOhms = 1mV.
This is too low to drive a comparator directely, therefore you need to amplify it.
For standard comparators a threshold voltage of 50mV should be sufficient.
To get 50mV output from 1mV input you need a gain of 50.
It seems you have single supply only (no negative supply)
Look for an Opamp:
* rail to rail output
* input common mode voltage should go down to GND
* input offset voltage < 100uV
Connect the both opamp circuit inputs (signal as well as GND reference) directely at the 20mOhms shunt with extra traces.
Klaus