How to see log from Arduino IDE?

The Serial.print statements show up on whatever terminal program is opened and connected. Their can be only one open at a time, but it could the Arduino IDE built in monitor (ctl-shft-m) or an external terminal program, eg putty, or some other program you wrote that opened the com/serial port the Arduino is on.

Remember though - only 1 can be open at a time. If you use an external program you MUST close the com port before attempting to download a new version of your sketch. If you use the built in monitor/terminal it will automatically do this for you.

It took me ages to realize that instead of remembering Ctrl-Shft-M you can just click on the little magnifying glass in the upper right of the IDE interface. Wait until after your script uploads though, it won't do anything while compiling or uploading your script.