Clipped glass corners are the easiest method for me. Otherwise, a bit of practice with your lenses will do the trick. And if you can use rear tilts or swings instead of front, do that. Ultrawide lenses are a pain in the butt, esp if the bellows gets pinched into the image area - a risk beginners are often unaware of. As Maris suggests, you could look through the lens itself, backwards, but that might not be such a good idea if you have you camera peering over the edge of a cliff (like I have often done), or on the side of the road with a big semi coming, pulling a mobile home. Anyway, with a bit of practice, one simply gets accustomed to what one's chosen lenses will or won't do. Fussing with published image circle data might be helpful when one is shopping for a lens in the first place, but it won't help much in the real world when various camera movements potentially come into play in the same shot.