This may seem like a fairly basic question. But here we go!

I want to performance test a 436MHz high gain antenna. My plan is to
construct a simple dipole with a 1:1 balun for 436MHz as a reference
antenna
and construct another dipole with a 1:1 balun to receive the test signal,
measure it with a diode detector and a milli-amp meter (field strength
meter) at the shack. Do the calculation and have the antenna gain.
This seems to me to be fairly straight forward, but has anyone carried out
similar measurements and concur with the approach or are there are there
traps and pit falls that I need to be aware of. Or is their simply a
better
way?

You may want to use a small directional antenna on both ends. The ARRL
handbook gives a standard refferance gain antenna that you can use on the
end that you are going to replace with the antenna you are building. The
antenna is suspose to have a certain ammount of gain if constructed
exectally as given.
The directiona antennas will help eliminate some reflections. Also make
sure they are far enough apart to be out of the near field.