Of course, the final authorities are our native speakers, but I'll share what I've found. From what I can tell, the difference when 'ver" is a prefix is sometimes very subtle and other times not so subtle. If there is a general rule, and i'm not sure there is, it might be that "ver" makes it a transitive verb, i.e. one that has an object. For example, mieten and vermieten can both mean to rent, but vermieten also means "rent out" so if you had a sentence where a room was being rented out to someone, "vermieten" would probably be the correct verb to use. Basically, I do a Google translate and if the distinction still isn't clear, do a search and you will likely find a grammar article on it somewhere. If that doesn't work for you, then you will have to ask our native speaker, Paul.