Perhaps you could use a multimeter and a simple analog-to-serial sketch, so you can directly check voltage and analog reads.
There should be an example in the ide, anyway it's as simple as:
void setup() {
Serial.begin(9600); // or whatever speed you prefer
}
void loop() {
int value;
value = analogRead(0); // use the correct number for your hardware
Serial.println(value);
delay(1000);
}
Upload and open the serial monitor, then keep watching the multimeter readings and the serial values on the screen.
Hope this helps...