Just a little point re diameter...don't forget any metal can weigh the same as gold at a given diameter. Namely, the depth is also important.
If it's got a nice quality milled edge and you can rule out electroplating by a combination of correct weight/diameter/thickness, then you're half way there. The other half would be design irregularities, and brass, which doesn't tone, or even look like gold unless it's highly polished.
A good set of well-calibrated scales will certainly help put your mind at rest.
You will seldom find 'modern' gold overweight, if ever. I'm no gold expert, but I've never seen an overweight sovereign.