option1:
send the multiview to NDI
for example, put the multiview to Output 2 and turn on NDI.
then you can open NDI STUDIO MONITOR and see the multiview there

option2:
or if you have another machine, open vMix on another PC/Laptop connected to same network, and
then add NDI input and then select the multiview so you can monitor it there

option3:
if you don't have another machine or you have no NDI Studio Monitor installed, another option is to open another instance of vMix and you can add NDI input there and select your multiview or add camera input, and tick Legacy, and select External Output 2 as your camera input

option4:
Just open the input settings of your multiview input and it will be in window mode, so you can view it as a separate window and place it in a corner or where ever you like.

i hope this helps.