Most supplier checks do not fail because the buyer asked nothing. They fail because the answers were never written down. A company name, a photograph of a workshop and a quotation arrive in a chat thread, the price looks reasonable, and the order is placed on material that would not satisfy anyone inside the business doing the buying. Verifying a supplier means deciding, before the order is placed, which claims you have tested, which you have accepted as risk, and which you could still test if the first shipment goes wrong.
Start with the entities, because they are usually three different ones. There is the company that signs the contract, the company that receives the deposit, and the company named as shipper on the export documents. Ask for all three in writing. The registration record gives a legal name, a number, an address and a stated business scope, and it takes minutes to compare against the quotation. An address that turns out to be an office building or a residential block is not proof of anything on its own, but it does mean the production question is still unanswered. So ask who owns the machines, who employs the operators, and whether any part of the order is subcontracted. Trading companies can be the right choice; the mistake is believing you are dealing with a factory when you are not.
Prefer questions whose answers can be checked against something other than the supplier's own word. Monthly capacity in units, the number of production lines, the name that appears on the export documents of a previous shipment, the lead time quoted to another buyer for the same item. Ask for the bank account details early, in writing, and check that the account name matches the contracting party; a request to pay a personal account, or an account in a third country introduced late in the process, is a reason to pause the transfer rather than a reason to cancel. References are worth only as much as the way you found them. A list of customers handed over by the supplier is a marketing document; a buyer you locate yourself, whose business you can confirm exists, is evidence.
Some patterns justify slowing down rather than refusing. A price far below every other quotation for the same specification. Samples sent from a different address than the one on the contract. A specification that shifts between emails without anyone saying so. Reluctance to allow a third-party inspection. Pressure to close before a public holiday starts. A supplier who answers a specific question with a brochure has not answered it, so ask again in writing and keep the reply. None of these settles the question, and all of them are cheaper to examine now than after a deposit has left the account.
Then write down what you have. A shortlist should carry the evidence next to each name: what was confirmed, how it was confirmed, and what remains an assumption. Keep that record short enough to read in five minutes, and update it whenever a claim is tested and either holds or falls apart. That record is what makes a first order a test rather than a gamble, and it is what an agency should hand you when the sourcing round closes.