What is voltage on digitial output when Arudino is supplied by USB? Is it 3.3V?
Which Arduino ?
What state is the pin in, HIGH or LOW ?
Arduino Uno r3 when pin is in state HIGH.
Slightly less than 5V then
The output voltage depends on the current being drawn out of the pin. The more current you are drawing the less the voltage. The data sheet of the processor in the Arduino has some figures for this. But basically keep the current down to 20mA or so and you get at least 4.2V.
But why do you want to know? This is digital electronics it is not much of an issue unless you are doing something fancy or wrong.