Karin,
You can order a small decibel reader online at fairly reasonable cost, or if you have a smart phone, there is a decibel reader app you can download in order to test the environment in question for sound levels. As I'm sure you realize, it's not just the decibel level, but the time of exposure that matters. One such graph showing permissible exposure levels and times is shown below. (FYI other studies and organizations might have slightly different numbers; this is just one such chart I found that popped up near the top of a Google search).
I often cut standard foam ear plugs in half and wear them when I go out to loud restaurants (but not night clubs or really loud bars -- those require full ear plugs imo) with friends. I can still hear fine but at the same time, I believe this strategy offers adequate protection. (No guarantees of course, but it works for me and my comfort level). The bonus is when cut in half, no one knows I'm wearing ear plugs. While I would not recommend doing this in very loud night clubs, wearing 1/2 ear plugs likely would be adequate protection in the type of environment you are describing.
If the restaurant turns out to be not loud enough to warrant any type of hearing protection, don't wear any plugs. Knowledgeable posters on another tinnitus board I have frequented in the past often adamantly warn against over protecting the ears. (They also abide by the "if you don't have to raise your voice to be heard, you should be fine without any protection" rule you mentioned in your post). Our ears/ hearing system actually need sound stimulation in order to remain healthy, and thus, wearing ear plugs in environments which don't require hearing protection can actually be counter-productive.
As to your other question about sensitivity, you might be experiencing hyperacusis, an over-sensitivity to certain frequency ranges of sound. A person with hyperacusis often has difficulty tolerating sounds which may seem unpleasantly loud to him or her, but not to others. Hyperacusis commonly occurs in tandem with tinnitus. I also experienced an increased sensitivity to sound with my tinnitus. Fortunately, my hyperacusis has overwhelmingly resolved and I am no longer overly sensitive to sound. My doctor stated that hyperacusis does tend to resolve over time, and that exposure to sound (in a safe, controlled setting) is a known method of treatment.
Exposure Time Guidelines
Accepted standards for recommended permissible exposure time for continuous time weighted average noise, according to
NIOSH and CDC, 2002. For every 3 dBAs over 85dBA, the permissible exposure time before possible damage can occur is cut in half.
The Noise Navigator®: a database of over 1700 noise sources.
Developed by Elliott Berger, MS, Senior Scientist with 3M Occupational Health and Environmental Safety Division.