You do not have to decide everything on first sight. If a question needs more thought than you want to give it now, flag it and move on.
How to flag
There is a flag button on the question. Pressing it marks the question. Pressing it again unflags it.

If you have turned on single-key shortcuts, F flags and unflags. See answering with the keyboard.
Finding your flagged questions again
The question list shows every question in the test and marks the flagged ones, so you can jump straight back to them rather than clicking through everything. The count of flagged questions is shown, so you know how many are outstanding.

What flagging does and does not do
- It does not change your answer. You can flag a question you have answered, a question you have not, or one you answered and want to reconsider.
- It does not tell the employer anything. Flags are a working tool for you and are not part of what is reported.
- It does not stop the clock or hold the question open. A flagged question you never return to is simply left as it stands when time runs out.
Using flags well
The useful habit on a timed test is to answer everything once, quickly, flagging anything you are not confident about, then spend whatever time is left on the flags. That way you never lose marks to questions you never reached, which is the more common way of running out of time.
Remember that an unanswered question is scored as an unanswered question. If you are running low on time and a flagged question is still blank, an answer you are unsure of is worth more than no answer.