Measuring the specific gravity with a hydrometer might get you close.
Titrating - neutralizing a specific amount of the unknown strength acid against
a known concentration of a base, using an indicator such as phenolpthalein
solution to determine when you hit neutral, measuring the exact amount of base
needed and then doing the calculations - will give you an exact number.

Many solutions have what is called an azeotropic point, or constant boiling
mixture. When you boil them, either the acid or the water will come off faster
until that point is reached. I couldn't find it in my chemical handbook (it
should be there) and can't remember where it is for nitric acid, but believe it
is well below the fuming or the 70 percent levels.

Hope this helps.

John Martin