We could use:
https://github.com/mozilla/pdf.js/blob/c3d08580627471432a6ad0b2fdb948df64126afa/src/display/api.js#L929

and look at bit 4.

The following pdf:
https://orimi.com/pdf-test.pdf
is not editable in neither Acrobat nor Edge nor Foxit and nor Okular.
if (this._spreadMode === SpreadMode.NONE) {
          break; 
        }
        const parity = this._spreadMode - 1;
        if (previous && currentPageNumber % 2 !== parity) {
          break; 
        } else if (!previous && currentPageNumber % 2 === parity) {
          break; 
        }
        const { views } = this._getVisiblePages(),
          expectedId = previous ? currentPageNumber - 1 : currentPageNumber + 1;
        for (const { id, percent, widthPercent } of views) {
          if (id !== expectedId) {
            continue;
          }
          if (percent > 0 && widthPercent === 100) {
            return 2;
          }
          break;
        }
        break;
      }
    }
    return 1;
  }
  /**
   * Go to the next page, taking scroll/spread-modes into account.
   * @returns {boolean} Whether navigation occured.
   */
  nextPage() {