Where can I see outputs from such instruction in an ino file ?
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.