That’s vendor-dependant. You might also discover that there are undocumented capabilities present on the UI and not included in the TWAIN header, or post-processing features that aren’t included in the TWAIN driver.
Most standard capabilities present in TWAIN are more or less easily discoverable in the UI as they fit the description given in the TWAIN specification.
In other case, you’ll have to either request information from the vendor or try some reverse engineering.