Look for the wire gauge and length and do the math. The gauge is usually printed on the cable in a format similar to 28 AWG 1 pair / 24 AWG 2C which means that the data wires are 28 gauge and the power wires are 24 gauge. Cheap cables are 28/28 which is too thin. When in doubt use a shorter heavier gauge cable.
Barring USB type C power delivery, Qualcomm's Quick Charge, Etc. Devices generally expect the USB voltage to be in-spec at the device itself for proper charging. The spec is 5 volts + or - 5% so 4.75–5.25 volts. Remember a modern lithium ion cell in a smartphone charges to 4.35 or 4.4 volts, only .35 volts overhead available for the charging circuit to work with worst case.
Example: I am using a 2 meter USB cable with 22AWG power wires.
22AWG is roughly .05 ohms per meter per AWG American Wire Gauge Diameter and Resistance
4 meters round trip.
.2 ohms total
.4 volts drop in the cable at 2 amps.
Given a 2 amp capable adapter and device the adapter would have to put out 5.15 volts to have the minimum available at the device at 2 amps. If you have this cable and an adapter that puts out exactly 5.0 volts the device likely will not charge at full speed.
The adapter that I am using right now is the stock one for the Samsung Galaxy S5. It puts out 5.3 volts no load and the voltage goes up to 5.6 volts at 2 amps, thus with this specific cable the voltage at the device will be in spec at anything above no load.
