Social Proof could mean: - You are a Serial Entrepreneur - A famous Business Angel invested in your company - You graduated from a University with a great Alumni Network - etc.

Basically if a potential investor evaluates your Market, Traction or Business Model, then you already lost. If a potential investor invests, because he thinks other investors want to invest, then this is Social Proof.

For example you are talking one hour with a potential investor:

If you talk 90 % of the time about who else is investing and about how to structure the investment round, then you have Social Proof. If you instead talk 90 % of the time about how big the market is, what your revenue model will be any why your churn rate only decreased 2 % in the last month, then you don't have Social Proof.

Also if the potential investors talks about making the investment bigger to increase your run rate and increase your leverage for a Series B, you have Social Proof. If instead the potential investors is asking you if you can decrease your burn rate or recommends you to raise a smaller amount, then you don't have Social Proof.

EDIT: Added one paragraph

But to get those success you need social proof.

It's a hard problem.