

明日は雨が降るかな。どうだろうね。
彼、本当に来るの?うーん、どうだろう。
新しい店、おいしいかな。どうだろう、行ってみないとわからないね。
"どうだろう" is in the lesson "Verbs 20" (Level 43), along with 10 other useful words. LinDuo has 180 topic lessons like this one, 2,379 words in total. Every word has an illustration, a transcription and pronunciation in a female and a male voice. Just 10–15 minutes a day is enough to learn them all. Free, and works offline.