Vishay TCST2300 Testing (Transmissive Optical Sensor with Phototransistor Outpu)
You are looking at the absolute maximum ratings, not the recommended operating conditions. This is a common mistake for those new to this stuff.

The 70V is the absolute maximum \$V_{CEO}\$ (Collector to emitter voltage with base open) and the 7V is the absolute maximum \$V_{ECO}\$, the reverse voltage (maximum voltage at the emitter with respect to the collector with base open).

If you measure across the collector to emitter (positive lead to collector) you should see this voltage decrease as light is applied to the detector assuming a resistive load; just what voltage that is I have no idea of (probably somewhere between 5 and 12V with no light at the detector).

Likewise you could measure across the detector photodiode pins which should increase in voltage (from anode to cathode) as more light is detected.

The test circuit shows the output from the emitter but there is no reason it could not be at the collector (which is why I suggest measuring across the device).

Once you have measured across the device you could measure the emitter with respect to ground and at the collector with respect to ground to see where the output is actually taken from.