you could try and shoot something down a line..like the edge of a table.. so see where the focal plane is. Then you can examine if that is sharp enough for you or if it falls below what you believe is correct? Ideally you'd use a tripod and the timer for shutter-release.
I can see that it was taken outdoors - any chance that handshake and/or even the most minuscule wind or vibration may have made it less than optimally sharp do you think?
51 focus points..I can barely keep up with the three my D40 has...lol.
Good luck with the investigation!