It can be very difficult until you try, but generally it is about being in tune with your body and emotions. If you start to feel yourself getting tired or weary, more emotional or frustrated than normal, it can be a sign that you are overworked and starting to burn out.

Effectively, your study/work combination will be the equivalent of just over a full-time job. And that can be a lot of work, especially if you still have assignments and studying outside of that time.

The other way to look at it is to work out a map of your week. I learnt to do this in a time management seminar. It is normally done in a table but will try to outline the basics here the best I can:

There are 168 hours in a week.
If you get 8 hours sleep a night (recommended), and take out 5 hours daily (seen as an average) for personal care, cooking, cleaning, eating etc. that leaves 77 hours. And these are just estimates so you may need to modify it for your own routines.

From there, attempt to map your current activities - travel, clases, work, study/assignment time, leisure time, family time etc. and see what is left.

It seems like a very rigid way to do it, but you very quickly can see how the hours add up. If you find that you won't have enough time to get things done easily, allowing for enough leisure/rest time to stay focused and a few 'spare' hours for things that come up unexpectedly or take longer, then you probably need to re-evaluate whether you can take on that much.

I really hope this is making sense! For me it was a great tool but is hard to explain like this.