Generally, you can't unless the transistor has the base connection available externally.
What is the optocoupler?
Does your multimeter have a transistor test option?
Even if the base pin isn't there, you could try connecting the + and - probes of a DMM to the collector and emitter pins respectively, setting the DMM to an Ohms range, passing a small current through the opto LED and looking for a change in the meter reading.