Optocoupler 4N25 saturated CTR
You don't look at any curve - you learn what saturated means. The general definition is that the collector-emitter voltage is less than the base-emitter voltage. Since the base of a 4N25 is available, you measure that. For most NPN silicon transistors, you can assume a base-emitter voltage at currents of 1 mA or more to be about 0.7 volts.

Your graph is useful, but not for the curve. See the conditions at the upper left? It shows that saturation (in this case) was defined as collector-emitter voltage of 0.4. This is a decent rough number, but could quite possibly get as low as 0.1 or less for high LED currents and large load resistors.