You can check the resistance of the bed heater or the hotend heater. They will be tiny, like 10Ohms or less. But find a spec sheet for them to be sure. 12V and 24V versions have different values.
The thermocouples are harder. They are all about the same near room temperature. You can just turn on the printer and see the reported temps. Something that isn’t broken would be in the room temperature range (23Cish). But, for example, if you picked the wrong kind of thermistor in your firmware settings, you wouldn’t be able to tell at room temperature. Even at 100C (which is conveniently where water boils) they aren’t much different. But the good news is, it doesn’t really matter. If you have the wrong config, and you add 5C to the temperature, it will go 5C higher. If that is 195C or 200C… Who cares? The important thing is how the plastic works and you will be tuning temps later for that.
To test the extruder motor, you will need to heat the hot end up to at least 150C.
If you have been a little careful configuring marlin, then you are probably ready for a heat test. Choose a program with a temperature graph, like pronterface or octoprint and set the temp on the bed to 60C. Watch it climb, look out for smoke, keep your hand on the power switch (not literally, but know where it is). Then feel that it is warm. You can measure it with a thermometer. It doesn’t have to be perfect. Turn it off.
Then do the same with the hot end. Start at 150C with no plastic inside. Watch it climb and stop at 150C. Then go to 195C and add some plastic. You can test extrusion. The controllers have an extrude button with some mm. Then turn it off and see that it drops on the graph.
Then onto the tuning. PID tuning, heater tuning, bed leveling, extrusion steps, all that good stuff.