It is the most direct way of checking for gravity segregation in the master alloy. A whole cross section would be best. It depends on how much you plan to do and whether you will be able to build up experience with the microstructure. Metallography will also be needed to confirm that the solder you produce is eutectic as required. Metallographpy of the whole cross section will check both for average conmposition and gravity segregation.
Chemical analysis is also good if available.
Finally, density measurement has proved more accurate and faster than chemical analysis in PbSn solder, once some experience was built up. It is simply weighing pieces in and out of water as in high school. If it essential to get as close to pure eutectic as posibble, a deviation in composition might be necessary.