A front company differs from a shell in that it trades. It has an office, staff, a bank account and a plausible line of business, and that ordinary activity is what makes its unusual transactions hard to pick out. A trading company that genuinely imports machinery for years is a far better cover for one restricted consignment than an entity that has never done anything.
The detection problem is therefore not “is this company real” but “is the pattern of this company’s business consistent with what it says it does”. That is why so many published indicators are comparative: volume against declared capacity, product against declared sector, counterparties against declared market.