A lot of the time, the spacings for such pokes are so precise that it's very impractical to specifically aim for them. You have to rely on knowing when your opponent shields, for how long, and whether or not they tilt their shield properly. The most practical shield pokes that are worth working on are ones that involve attacking platforms, usually following their tech.
If you have 20XX, you can place a "punching bag" CPU on different platforms and lock them in permanent full shield by pressing Down on the D-pad and then L+Left. From here you can toggle the collision bubbles (X+Right) to see where they have vulnerabilities. Most characters have some spots that you can poke at while they are at 100% shield. The hardest part is obviously getting that hyper precise spacing to allow your hitboxes to make contact with their yellow collision bubble before making contact with their shield. Some pokes are easier than others, for example Marth's tipper F-smash hitting people's toes on Battlefield and even Yoshi's. The problem is that this is easily countered by someone tilting their shield to cover their weak spots.
In a vast majority of cases, countering shields with a different option will be more effective. Shield pokes are there still to reward good spacing and punish bad shield behavior. What's most worth practicing is how to follow-up on common shield pokes.
