You can test your value proposition in a few ways:

A/B Testing

The best way is to craft 2 candidates (or more, if you have tons of traffic) and split test them. Ideally you would measure sales conversions (for most accurate results), but if that is not possible lead conversions or even clickthroughs will do.

Pay Per Click Advertising

A fast and cheap way to go about it is using Google AdWords or Facebook ads.

Basically you would split test ads with different value propositions, targeting the same customer. The ad with higher CTR (clickthrough rate) is obviously a better attention grabber and interest generator, although it doesn't necessarily mean higher sales conversions.