A lot of planes, even discontinued, you can go to the manufacturers website and look up the instruction manual. also if you can use Google you can find som CG calculators online where you type in some measurements from your plane and it gives you a CG location. Most experienced pilots will take a typical high wing and balance it at 25% of the wing chord (length not span) back from the leading edge for an initial flight. maybe as far back as 33% if it's a hotter aerobatic model but even on those 25% is pretty safe for a starting point.

Also if you tell us what you have maybe someone here has one and can tell you exactly where it's suposed to be.