I am afraid that external validity is not a fancy number you can calculate, but is how much you can generalize the results of your experiment (probably in a laboratory) to the external world, i.e. other situations (generalization across situations), other people (generlization across people) and so on.
So, the best way could be to conduct and replicate your experiment in different situations and measure the effect. If it is nearly the same across people/situations, you have a high external validity.
I am afraid that external validity is not a fancy number you can calculate, but is how much you can generalize the results of your experiment (probably in a laboratory) to the external world, i.e. other situations (generalization across situations), other people (generlization across people) and so on.
So, the best way could be to conduct and replicate your experiment in different situations and measure the effect. If it is nearly the same across people/situations, you have a high external validity.