This is from a NEAP past paper book (Biology)
Evaluate - means to make a judgement based on criteria; determine the value of. In this type of question you are required to make a judgements about people, ides and possible actions. You will need to make an evaluation statement based on certain criteria that you have developed.
Assess - means to make a judgement of value, quality, outcomes, results or size. You will need to make points for and against and then an assessment statement based on certain criteria that you have developed.
Hope this helps! =)