Use a quick magnitude and unit check before presenting the answer. Ask whether the number is in the right order of magnitude and whether the units match what the question asked for.

Compare it with a simple benchmark from the case. If revenue is 100 million, a calculated cost of 8 billion should immediately trigger a recheck.

The sanity check should take seconds. It is not a second full calculation. If useful, this guide on case interview mental math gives more detail.