Oh hi, Darlorth! Welcome
Like with other objects, you can inspect any machine (right-click on it or use the kb-based examine method) and read its stats in the info window. That window lists its armor and resistances, and if you're not sure what those values mean, there is context help for each (click on the stat). You may not actually have anything that is capable of achieving what you want to do in that case--depends on the Garrison Access and how deep you are.
Garrisons are not that easy to destroy, not to mention doing so is really bad for your alert, so you're best avoiding them whenever possible, unless you have some other specific strategy in which destroying it will actually be helpful in some way....
But anyway, the stats are all static so once you're more familiar with the various weapons and different machines you'll start to have a general idea of what you can blow up in a given situation and how difficult it will be. Again though, basically stay away from those if you can help it! There are strategies that involve attacking (or more likely hacking) them, but you'll get to that later
(Technically if you go inside one and use a RIF Installer, you'll be able to very easily lock any Garrison Access machines you come across, which is pretty handy among the other benefits you obtain... but that's probably a more advanced strategy overall.)
