How do I determine whether my EEG signal needs denoising? I recently start to work on sleep study.
For my research i download sleep EEG data from physionet. The EEG data has 100 HZ sampling rate and was recorded from 2 bipolar EEG site.  

When i start the preprocessing stage, i encounter a simple problem, how should i know if my signal has an artifact or noise? 

It should be noted, based on nyquist theorem and my signal's sampling rate, the maximum frequency of my signal is 50 HZ, so i did not filter unnecessary EEG frequency.
In general i only used a simple notch filter at 50 HZ, and used simple threshold method in order to remove the epochs that were grossly contaminated by muscle and/or eye movement artifacts. 

Back to the main question, how should i know if i need to uses more complicated method for removing EMG or EOG artifact from my signal?
Your recording necessarily has noise. That's a property of any physical apparatus, and you are recording the activity of noisy neurons. The other property of noise is that, by definition, it will average out. So don't do anything about that. 100Hz is quite a low frequency. Artifacts (e.g. blinks, but I doubt you would have that in sleeping subjects) would typically have a higher frequency than that so (1) it's unlikely it has been recorded by your system at all, (2) you can low-pass filter your signal to remove both the 50Hz artifact and possible other artifacts. Other artifacts such as sleeping position would have very low frequencies. So you can high-pass your signal to get rid of them. I'm not familiar with sleep EEGs but I would not worry too much about it. You can also check few papers and see how they filtered their signal.