"Is it one o'clock?"
です is put at the end of a sentence to make it more polite and particles aren't placed before it. For this case, it's also used to translate the "is it" part of the sentence. The か makes the sentence a question. If there was something else added to the sentence such as 今 (ima), you could use a は particle (for example: 今は一時ですか｡ [ima wa ichi ji desu ka]). But for the sentence you're given, there isn't anything to tag a particle with, and is therefore not needed.
I have a question here.
Would it be common / normal for people to say "今は一時ですか?" and not just "一時ですか"? Also, say the context of the conversation is different and people are talking about some other event...like maybe someone arriving or some scheduled event, or some entry in a list like for people signing up for appointment time slots. In this context would "一時ですか" be understood to be about whatever you're talking about, whereas adding 今は specifies that you're asking what time it is currently?
I'm still wrapping my mind around how Japanese uses topics and implied subjects, and although I have an impression of how these things work, I want to make sure that I'm getting it right.
if you are asking time , we don't normally use "今は一時ですか?" we use, "もう一時ですか?" is it one o'clock yet? or ’もう一時になりましたか？’ is it already one o'clock? or ’いま何時ですか？もう一時になりましたか？’what time is it? is it already one o'clock?
you can use "一時ですか"? or '"一時ですね?”to confirm your doctor's appointment or other appointment time, such as lunch appointment.
"今は一時です" is grammatically correct, but, we don't use it.. we use, ’ただ今の時間は一時です。’ current time is at one o'clock... or 'もう一時です。it is already at one o'clock.. on simply, '今,一時です', right now, it is one o'clock.