The correct answer is B) check the lube oil temperature. The most practical method of determining the condition of a shaft bearing while the shaft is in operation is to check the lube oil temperature. This is because an increase in the lube oil temperature can indicate an issue with the bearing, such as increased friction or wear. Monitoring the lube oil temperature provides a real-time assessment of the bearing's condition without the need to shut down the system or perform additional tests. The other options are not as practical or effective for determining the bearing condition while the shaft is in operation. Performing a carbon blot test (A) or checking the lube oil viscosity (C) would require taking an oil sample and conducting laboratory analysis, which is not as immediate or practical. Visually inspecting the bearing (D) is not possible without disassembling the system, which would interrupt the operation.