If you are talking to a station using a repeater, how would you find out if you could communicate using simplex instead?

A repeater receives a signal on one frequency (the input) and simultaneously retransmits it on another (the output). When you hear another station through a repeater, you are listening to the repeater's high-power output, not the other station's direct signal. To determine if you are close enough to talk directly (simplex) without the repeater, you should listen on the repeater's input frequency. Many modern radios have a 'reverse' button exactly for this purpose. If you can clearly hear the other station's direct transmission on the input frequency, their signal is strong enough to reach you directly. Once direct reception is confirmed, both stations should agree to switch to a designated simplex frequency, freeing up the shared repeater for others who actually need its extended range.