There have been a few circumstances where I've had to email an authority at my school and didn't know whether they had a PhD (e.g. An instructor of mine who I knew had been a PhD candidate a few years earlier, but perhaps graduated since then). In these cases, should I address the person as Dr.?
In my opinion it strongly depends on the culture. In Switzerland we use solely "Herr"/"Frau" (Mr/Mrs) to address even professors. If you have a look at the history of the country you get an idea why titles of any kind are somewhat frowned upon. However, in Germany the situation is already different and at least under certain circumstances the title is to be used in the address. In Austria it's even sort of a must as I got to hear. At least some people were somewhat sulky when I failed to do so :)
I do not know what role titles play in other places of the world.
If you don't know who the person is then there is no point of insult in normal greeting; however if you try this with non-phd person it may appear that you are trying to butter the person or show you in lesser light; as a person of lower intellectual capability; as everybody appears to be a dr to you.
Lastly even if you know a person has the phd; it shouldn't insult him/her if they are your coworkers or boss; remember mgmnt in software cos usually encourage usage of first name and encourage casual informal relationships so as to foster stronger working ties (you don't want your coworker to be afraid or in awe of you so that they don't use their brains or agree blindly to whatever you say); as in a professional situation you all have the same goal and job; while in academics phd's ,post-doctoral guys are the knowledge searchers creators and you are the learners; it may insult them if they are your teachers/professors as that relationship is much different than a Professional relationship.
Well, I have a letter envelope from the D. E. Knuth addressing me as "Professor". Which is several steps above what I could actually be claiming. He (or his secretary) probably preferred erring on the safe side over guessing whether my lack of using titles was due to modesty.
Now putting suspected titles on the envelope and leaving them off in the communication itself may actually be not a bad idea since then any possible embarrassment does not accumulate with multiple mentions but you still bring across that you consider the recipient capable/likely to have some degree.
At any rate: if you cannot find any titles in material you are able to find of the person in question, chances are that he/she is not all that obsessive about having his titles attached to his name whenever possible even in case he may have claim to them.
In the UK, it is common to simply address academics by their name, without a title. "Dear Firstname" if you know them personally, or "Dear Firstname Lastname" to be slightly more formal or if e-mailing someone outside of your department. I would use however "professor" or "Dr" in more formal correspondence (such as appeals or anything by post) assuming I was certain of their title, but there is no harm in omitting it.
So to answer your question: I would in most circumstances omit the title even if I knew it, and it is common in the UK to do so, though there is no harm in using it. If you don't know, "Dear Name Name" will suffice.
Address the person by her academic role if she has one. For example, if she is a a professor, say "Dear Professor" or "Hi Professor". If she is an instructor but not a professor try "Dear Instructor".
I did this with my daily supervisor who was a professor but did not have a PhD in my early correspondence with him and it worked fine.