I would like to say that you need to have an expert come and check out the roof. You may be able to do a good evaluation yourself. There are a number of people that would be willing to take a look at your situation. Some will do it for free, others for a nominal fee.
These contractors will probably be well qualified to check some areas of concern. You can make a pretty good evaluation yourself. Does the wood look different? Has the roof started to look like it may be sagging? Is there some unusual staining in the ceiling? A bad roof usually shows you the problem; you just need to be aware of the symptoms. Common sense will generally be a guide to whether or not you have an issue that needs attention.
Yeah, I think I have a problem. Now we can get to the solution, call someone who can fix the problem. Get a qualified opinion and an estimate for the repairs; if you have a dry rot condition it will only get worse.