To express probability, you can use the construction ~(으)ㄹ 거예요 :

다음 주에 시험이 있을 거예요 = Next week, there will probably be a test.

(다음 = next. 주 = week. 에 = particle. 시험 = test. 이 = particle. 있을 거예요 = will probably be)

The construction ~(으)ㄹ 거예요 doesn’t just express probability, it expresses the likelihood of events happening in the future :

거기서 집을 지을 거예요 = A house will probably be built there (in the near or distant future)

(거기서 = there. 집 = house. 을 = particle. 지을 거예요 = will probably build)

Sometimes certain adverbs like 아마 are added to emphasize how probable something will happen in the future :

아마 비가 올 거예요 = It’ll probably rain (literally the rain will come)

(아마 = probably. 비 = rain. 가 = particle. 올 거예요 = probably will come)

The construction ~(으)ㄹ 거예요 is often mistaken with the construction ~(으)ㄹ 게요 :

내일 돈을 보낼 게요 = I’ll send the money tomorrow.

(내일 = tomorrow. 돈 = money. 을 = particle. 보낼 게요 = will send)

With ~(으)ㄹ 게요, the timeline in which the events occur happen in the future, but unlike ~(으)ㄹ 거예요, there is no sense of probability.

Both constructions can be used to state the opposite of something happening in the future :

아마 선물을 안 살 거예요 = I will probably not buy a present

(아마 = probably. 선물 = present. 을 = particle. 안 = not. 살 거예요 = probably will buy)

선물을 안 살 게요 = I will not buy a present

(선물 = present. 을 = particle. 안 = not. 살 게요 = will buy)