You will find out in time the truth of what I say.

what I say 是名词性从句，名词性从句的作用相当于名词。what I say = my words。

You will find out in time the truth of what I say.=

You will find out in time the truth of my words.

what I say 使用一般现在时，意味着“我这人说话真实可靠，向来都是这样”。若改为 what I said，则说明是这次说过的话。