You can use 1:1 (volume by volume) Nitric acid to dissolve the whole EN deposit. Then using ICP to check the Phosphorus and Nickel content. Therefore you may calculate the percentage of phosphorus in the total Ni+Phosphorus. It does not matter to dissolve some of the iron from the substrate because you just need to know the content of nickel and phosphorus. Usually we present the P-content in percentage. This is the most accurate method.
The alternative method (quickest) is to use SEM/EDX to check the composition of the plating deposit. But it is not accurate enough because the data cannot represent the total plating deposit (sampling size is small). However, it is the quickest way to check the P-content without destroying the EN deposit.