I use a Canon 700D, and my focus is always just a little bit off. What is the best tethering device I could use to preview on a bigger screen?
I'm not sure that tethering is going to help, unless you are specifically manually focusing and just can't quite nail it. You can also try a better orientation of your subject to be more in line with the camera's sensor plane. Also, it looks like your camera has the ability to perform focus micro-adjustment for a given lens. You can try this by printing out your own device, here: http://bobatkins.com/photography/technical/focus_testing.html . Canon themselves have an article on focusing in general as well, here: https://www.learn.usa.canon.com/app/pdfs/quickguides/CDLC_Accurate_EOS_AF_QuickGuide.pdf .
Update: Sorry, it looks like I was incorrect, the 700D does not apparently have micro-focus adjustment, based on this excellent response here, by https://photo.stackexchange.com/users/15871/michael-clark: Can the 700D perform microfocus adjustment?