In a dry skillet, toast the caraway seeds over medium-high heat until fragrant and slightly darker, about 30 seconds (watch carefully; this happens fast). Transfer to a small bowl to cool. Once cool, rub the seeds between your fingers to crush them slightly.
In a large bowl, whisk together the vinegar, olive oil, caraway seeds, and dill. Add the cucumbers, radishes, and fennel bulb. Toss well with the dressing. Transfer to a serving platter and garnish with the reserved fennel fronds.