Can you tell by taking a person's pulse if they have atrial fib?

Yes. Atrial fibrillation, by definition, is an irregularity in the heartbeat, and therefore is detectable by manual palpation of someone's pulse. It should always be confirmed with an EKG tracing, because there are other causes of pulse irregularity which occasionally can be difficult to distinguish on physical exam, and which have different implications for therapy from atrial fib.
Yes. You can feel if the pulse is irregular but it is not necessarily A fib as there are many other things that can cause an irregular pulse.
Not Always. Someone with af often has an irregular pulse; however, the irregularity can sometimes be too subtle to pick up by just feeling the pulse. Other rhythms can also cause irregularity and not be af. An ECG is essential to make the diagnosis.