I have a drill that if I can explain correctly will give you instant feedback if you take it too inside or if you happen to get too much underplane coming down as well. Hopefully you can visualize what I'm saying.
First place a club, say a 5 or 6 iron on the ground on your target line, pre. I place the club on the ground where the head is at the back end. Around the ferul/hosel area I take my setup with the clubhead as close to the club on the ground, no further away than an inch or two. Once I take the setup, I take an old shaft and stick it in the ground again an inch or two under my club, on the same angle as my club is at setup. then move up toward the front end of the club and tee a ball up (always use a tee and don't worry if you tee it up higher than normal, it's just a drill), again with the clubhead as close as possible to the shaft on the ground. Then the idea is to swing and keep your club above the shaft you stuck in the ground at the rear end of your target shaft...if you come too inside either going back or coming down you will hit the shaft. I don't know if you can visualize what I'm saying but if so it will let you know right away if you take the club too inside.