There are a variety of factors which determine whether a site is good for guest posting.
The main factors to make this distinction are:
Online authority and whether the site will appear in a search and give you the exposure you're looking for.
Whether there is a large audience the site will help you to reach.
A target audience that contains the demographics you're aiming to reach.
If the site contains topics and content which are related to your industry.
If these factors are tended to you will know it is a good guest blogging site.