。
This time tomorrow I'll be taking the exam.
Literal
Tomorrow [possessive-の] this-about-time, I [topic-は] exam [object-を] be-receiving probably.
受ける takes を for what you 'receive' — in this case an exam (受ける + 試験 = 'sit/take an exam'). The ~ている future-progressive again: 'will be [in the middle of] taking.' 今ごろ is the hiragana rendering of 今頃, both extremely common.