If your DMM has a low current port designed for this purpose, you can safely measure Resistance in an energized circuit. When doing so, ensure all connections are secure and all safety considerations are observed before proceeding. Also, always use the correct settings and probes for your measurements. Finally, remember that even when using a low current port to measure Resistance while the power is on, it is always best to proceed cautiously as there may be unexpected voltage spikes or other hazards in energized circuits.