Serum uric acid is a good marker of dehydration. You can use it in patients whose pre-dehydration weight is unknown.
In studies, serum uric acid correlated significantly with weight change (in dehydration) and increased significantly along with dehydration severity.
Reference
Pediatr Int. 2017 Nov;59(11):1151-1156. Uric acid and dehydration in children with gastroenteritis. https://www.ncbi.nlm.nih.gov/pubmed/28714223