Some general guidelines for guaging the worth of a conference would be something like:
1) Its relevance i.e. How relevant is it to your research area. Some conference just invite everyone and do not have specific goals.
2) It reputation. For instance everyone in the area of communications networks would know the worth of getting a paper published in IEEE Infocom or ACMs sigcomm.
3) A major and often over-looked factor is to have a look at the Technical Programme Committee of the conference. This is very important and the candidate should have a look at the members in the TPC before actually submitting. Once again, two conferences might have almost the same king of TPC in terms of whether whcih institutions have representations in them. However, one conference might have more seasoned researchers than the other even though they belong to the same universities.
