How can I check my defending troops to my base?

You can tap on your Garrison building and select the garrison option to see the set up (and change the order) of your APC's that will defend your base if attacked. You can also untick the 'join defense' option if you don't want them to defend (saves them from being wounded / killed)

After the APC's defend any extra troops that you have left over will also defend you. You can't see these here unfortunately. You can check in the Command Center Building to see how many troops you have overall and work out which ones are not in your APC's