でしょう can be a way to express probability, or a prediction or a guess.
あの人が加藤さんでしょう = That person over there is probably Mr. Katō.
(あの = that over there. ひと/人 = person. が = particle. かとう/加藤 = Katō. さん = Mr. でしょう = probably)
If you want to indicate the possibility of something or someone not being someone or something :
あの人日本人じゃないでしょう = That person over there is probably not a Japanese person.
(あの = That over there. ひと/人 = person. にほんじん/日本人 = Japanese person. じゃない = not. でしょう = probably)
When でしょう is used with nouns, like the examples above, just tack でしょう to the end of the sentence. When it is used with verbs, don’t conjugate the verbs. Leave them in the dictionary form.
田村さんは午後九時までに帰るでしょう = Mr. Tamura will probably return by 9 pm.
(たむら/田村 = Tamura. さん = Mr. は = particle. ごご/午後 = pm. く/九 = 9. じ/時 = o’clock. までに = by. かえる/帰る = return. でしょう = probably)
Like the sentence above, でしょう can also be used to express the probability of events that may or may not happen in the future. Even adjectives can be used with でしょう :
カリフォルニアは暖かいでしょう = It’s probably warm in California
(カリフォルニア = California. は = particle. あたたかい/暖かい = warm. でしょう = probably)
The sentence above used an い adjective, but な adjectives work as well :
その先生は親切じゃないでしょう = That teacher is probably not kind
(その = that. せんせい/先生 = teacher. は = particle. しんせつ/親切 = kind. じゃない = not. でしょう = probably)