Max/Msp and Arduino

I used a serial monitor in Arduino software, but when I use MAX/MSP, I don't know if the program is reading my sensor signal. Is there something that I can know MAX/MSP is reading the sensor and how much the signal is... ? Please help me. My arduino seems connected with MAX/MSP which has "on"light, but I don't know if it receives sensor power. ( I used MAXUINO test patch.)

There are a number of solutions, which depend on the operating system (Mac or PC). You should ensure that the serial port in Arduino (in Windows often COM3 or COM4) matches the one selected in MaxMSP, and the baudrate argument to the [serial] object matches your Serial.begin value; if your serial monitor is giving you the data you expect then the problem isn't an Arduino one.