Trace it to its origin, not to the relative who sent it. A gift needs a signed gift instrument plus evidence of how the giver lawfully accumulated the funds; a loan needs its agreement, security and repayment terms. Bank statements should show each transfer leaving the named source and arriving in the investor's account.
Draw the money map before opening accounts
Sketch every account the capital will pass through, with the holder's name against each step, and check that a stranger could follow it without explanation. Then collect the supporting records for each hop: sale contracts, tax filings, corporate distributions, exchange confirmations. Where a step cannot be evidenced — cash held for years, an informal family arrangement — raise it with counsel now, because restructuring the funding is possible before transfer and awkward afterwards.
Two habits make this trail far easier to build. The first is to keep the capital in accounts held solely by the investor from the moment it is identified, avoiding joint or family accounts through which other money also passes. The second is to collect the supporting document at the time of each transfer rather than afterwards, since banks and counterparties are cooperative in the week of a transaction and considerably less so a year later.
Where an arrangement genuinely cannot be evidenced, say so to counsel before the money moves, because the funding can usually be restructured beforehand and rarely afterwards.