So you have the outputs of the cd,cassette and turntable plugged into their own input channels of the mixer and the output of the mixer plugged into the input of the reel to reel.
There are a couple of things you could do to monitor the cd etc.
Plug your headphones into the mixer and select the required input channel on that channels monitor knob.
Plug your headphones into the reel to reel and monitor the input signal.
Connect a cable between the reel to reel headphone socket and a spare input channel on the mixer then plug your headphones into the mixer and turn up that channels monitor knob,this will allow you to do all the monitoring from the mixer.Note if you do this make sure you have that channel muted or you will get feedback.