I think you generally can tell whether a corset is an overbust or an underbust for most periods. Constructing or making a corset requires skill but just Google 'corset shapes underbust' and you'll find a lot of good reference information showing period corsets in detail. Here's a brief overview from bellaonline.com;
"One of the first things to consider with a corset is whether to get an overbust corset or an underbust one. As the names suggest, they each stop at different points up the torso. Overbust means that the garment extends up above the ribcage line and provides both support and coverage to the breasts. Women who have very tiny waists and large breasts will often find they have trouble finding overbust corsets that fit well. An underbust corset will rise to anywhere from the bottom of the ribs to just under the breasts but not cover them. Depending on the corset style, they may or may not offer some breast support. When paired with a top or bra made of a matching material, an underbust corset can fit comfortably and mimic the appearance of an overbust style. Underbust corsets can still cinch the waist but often aren’t as limiting to overall mobility as overbust styles or longlines. A "longline" corset is the name given to corsets that come very far down over the hips.
Historically, the corset has had a few shape variations, most often to accentuate or play down aspects of the human form so as to better shape it to match the clothing styles that were fashionable at the time. An hourglass corset is more balanced in coverage above and below the waist, with the most cinch provided right in the middle. Hourglass corsets curve very smoothly and produce a rounded waistline. This is the classic shape that most people are familiar with when they think of corsets.
If you have a lower waist, you may find that the corset shape more popular with the Edwardians will fit you better. This "straightfront" corset has an upper shape that is more triangular, and is sometimes likened to the shape of an ice cream cone turned upside down. The torso line drops down with a straighter shape and then rounds off into the hips. Conversely, if you are high-waisted, the classic wasp-waist corset might be better suited to your physique. This corset shape is more angular, dropping towards and away from the waist with straighter lines than the hourglass."
