Try your design as a physical part
A prototype gives you something to hold, compare and check. Use it to judge the shape, try the fit or decide what to change before making more.
What do you want to find out?
You might want to see how a design looks, check access to a feature or try it alongside another component. Tell us which question you want the prototype to answer.
For example, a model of an enclosure could help you check the position of its controls before changing the design.
Point out the features that matter
Identify the model version you want printed. Mark important dimensions, holes, gaps and surfaces that need to fit together. If it connects to another part, explain how you'll check the fit.
FDM and resin produce different surfaces and levels of detail. The material, printing direction, supports and shape all affect what you can learn from the prototype.
Use what you learn for the next version
Keep a note of what worked, what changed and what you want to check next. If the model needs adjusting, discuss CAD work before arranging another print.
A first print can help you make a design decision. Further checks are needed to establish final tolerances, durability or suitability for a safety-critical job.