How do I shop for a good tire? I put 26" slicks on my mountain bike so I could get good city riding speed and comfort without worrying about bad patches of road. The tread is mostly worn to having no pattern on both tires after about 750 km of use. Maybe that is typical I'm not sure. They were Kenda brand. The guy at the store showed me 2 models to choose from I took the ones that had higher inflation maximum so I could get them at max air pressure capacity for lower rolling resistance. 

How do I know I'm buying a good quality bicycle tire versus just an overpriced one? Or for that matter when is a cheap tire a good value or just poor quality?
Unfortunately the answer is the same as how you judge the quality of any product. Experience with the product, brand reputation, price (higher correlates somewhat with better performance) and searching for and reading reviews.

With respect to your current tires, they are probably not worn out. Not even cheap tires wear out after only 750kms. Road tires have a very shallow 'tread' pattern that is really just cosmetic - bike tires are narrow enough to displace water easily and do not need tread to grip tarmac or concrete surface. See How do I know when to replace my tires?. 

Also, you should know that higher inflation pressure does not necessarily mean lower rolling resistance on rough surfaces. See this answer and this GCN video.