Hildebrand's own words support the thesis. In the main text of his book, reference the people Hildebrand speaks about having cooperated with, and then look up their military records.
Or go to sources independent of Hildbrand's own words. In my book I lay out the primary sources very thoroughly that also supports your thesis. Go to pp. 179-180, footnote 4. Also pp. 191, f. 4; p. 198, f. 5; p. 205, f. 1; p. 212, f. 8; p. 213, ff. 3-4; p. 220, ff. 1-2; p. 221, f. 1.