If you have a handheld DMM that can measure millivolts, it is a straightforward task to perform a basic measurement.
In addition to the DMM, you will need a set of voltage probes with pointed/sharp tips (these will help penetrate the heatshrink without any damage). Or you can use alligator clips, again if you can slide the heatshrink back far enough.
Place the DMM (+) lead on one side of a connector/connection (could be an arming plug, ESC to motor bullet, battery to ESC); press thru the heatshink if necessary or carefully slide back the heatshrink if possible
Place the DMM (-) lead on the other side of the connector/connection.
Verify that you are reading 0.000VDC with the motor armed but not spinning.
Now, with the model restrained ...(an assistant is helpful), advance the throttle, placing current flowing thru the connection under test/measurement. Record the DMM reading, then turn off the motor (preventing any damage due to lack of airflow or increased temperature).
If you read a significant voltage across the connection, you have a bad contact/connection/soldering joint.
If you have telemetry or a clamp on current meter and can determine the current that was flowing when the voltage was measured, calculate the resistance by taking the voltage measurement (volts) and dividing by the current (amps)
As a secondary test, you can use a thermal probe (direct contact or non contact IR) and measure the temperature rise of the contact when current is flowing and compare it to when current is not flowing (ambient room temperature).
For example, if you had 50Amps flowing thru the connector/connection and you measured 100mVDC, the resistance would be 0.1VDC / 50A = 2 milliohms (2 thousandths of an ohm). That is a good number.
There is much more advance physics and theory behind connectors (really), but in general, if you are under 0.5VDC (500mVDC) under full load current (assuming gold connectors are used), then you should be OK for now.
Higher than that and you connector/connection is not really a connection, but in fact a resistor.
Think of it this way....
An ideal connection is 2 conductors held together by a bolt and nut. Lots of surface area, lots of contact pressure. No matter how much current you force thru it, you won't generate a voltage (drop), so there is no problem.
Unfortunately, connectors (bullets, Deans, pin/socket, blade/socket, etc) are NOT bolted together and therefore aren't perfect. Once you get too large of a voltage drop across, it's not a connector, it's a resistor.