The Mode 2 check (which is actually part of the basic GPWS not the predictive GPS-based EGPWS) MUST be carried out on the prescribed site as it requires a carefully surveyed slope of known gradient to verify the warning.
A note of caution, the entire crew should ensure they all understand the procedure perfectly (some site directions can be a bit obscure) and obey it to the letter. I know of a crew that didn't get on the exact heading and instrumentation showed they only missed the cumulo-granite by 180 feet!