”I read somewhere that the resistors have to be disconnected before you can test them”
you can always put your meter leads across a resistor ( not powered on) and check the reading. Sure there is a possibility that there could be another way for something else to be a secondary route between the two ends of the resistor. If there were ( for example another resistor in parallel) then the meter would read low, less resistance.
As Gerry noted the resistors might drift up in value over time, you can check to see if that is the case There can be nothing else in the circuit, such as another route, that connects the two test points across the resistor that could possibly raise the resistance value, another route can only lower the resistance..
So yes you can do some preliminary checking without unsoldering anything, Then if in doubt unsolder one leg to make sure nothing else is interfering.