how can anyone prove a rounded calculation is correct to the cfa institute in a computer based test without having his/her scratch paper scanned?

Got me.

I’m not saying CFAI does this, but one approach is to take the answer provided by the exam taker, round it to a set number of digits, and compare it with the official answer.

For example, a student answers n = 12.3456.

The system takes this answer, rounds it to 12.3 and compares it with the official answer of 12.3, and marks the answer as correct.

I thought the standard was to round to 4 digits. So you just do that…

Technically, you round depending on the number of significant digits in your calculation.

by the way, did you recently become a CFA? If so, congratulations!