DCF and EVA are future predicting models , but as u say that we need to know future for it , if i knew future then i wud have bought a truck load of stocks . But sadly that wud requide a boon from Gods after years of tapasya..
So what we do is try to predict the future and also know what factors are there that might affect my future projections negatively .
EVA and DCF are just models to do so