If it is still full and unopened it should still be fine. Metal cans are one of the best ways to store it. I don't think there is any way to test it other than running it, however. What you have is 10% nitro mix.
Origionally Cox Blue can fuel was 25% and racing fuel in the Red can was 30%. Of course this was the 50's but I'm suprised that they would drop the nitro so low. 10% nitro is marginal for decent performance in a 1/2A engine.
Cans marked "Flight Power" fuel contained the 15% blend. The race fuel was always advertised as 30%. Cox changed the titles on their fuel it seemed hard to know exactly what you were buying as far as percentages