By comparing the amount he has committed with the total cost of acquiring the business, supported by the valuation documents. The test is proportional; a majority stake in a modestly priced operating company, fully paid and committed, is ordinarily substantial.
Substantiality is his committed amount over the real acquisition cost
The Foreign Affairs Manual applies a proportionality test that weighs the amount invested against the total cost of purchasing an established enterprise. For a share purchase the relevant cost is the price for the business as a whole, established by the seller's financial statements, the equipment schedule and the negotiated agreement, plus any working capital the buyer must add. The buyer's committed amount is his 60% share of that price and any capital contribution he makes; the partner's 40% is not his investment and should not be counted as such.
A committed amount that represents the full price of his stake, placed in escrow pending visa issuance, meets the test comfortably where the business is modestly priced; the officer is more sceptical where the buyer has paid a small deposit with the balance financed by the seller against the company's own assets, since indebtedness secured on the enterprise is not the investor's capital at risk. Government fees for the visa application are small and listed by the State Department; the professional cost of due diligence and the business plan is the larger outlay.