The only one I came up with is Rodriguez-Robles (1998). Alternative perspectives on the diet of Gopher Snakes (Pituophis catenifer, Colubridae): literature records versus stomach contents of wild and museum specimens. Copeia 1998(2):463-466
I cannot think of a specific paper at the moment, but consider papers by Fabian Jaksic in the 70s and 80s—he did a LOT of diet analyses (mostly birds and mammals) based on different types of data.
Lots of this gets discussed in the stable isotope literature. Most of it is bird and mammal focused but still relevant to non-avian reptiles. Stable isotopes are a very underutilized with reptiles in my opinion. There are some good review papers out there but these should get you started.
Alexander, S. A., K. A. Hobson, C. L. Gratto-Trevor, and A. W. Diamond. 1996. Conventional and isotopic determinations of shorebird diets at an inland stopover: the importance of invertebrates and Potamogeton pectinatus tubers. Canadian Journal of Zoology 74:1057-1068.
Rush, S. A., K. Sash, J. Carroll, B. Palmer, and A. T. Fisk. 2014. Feeding Ecology of the Snake Community of the Red Hills Region Relative to Management for Northern Bobwhite: Assessing the Diet of Snakes Using Stable Isotopes. Copeia 2014:288-296.
Govender, Y., M. C. Muñoz, L. A. R. Camejo, A. R. Puente-Rolón, E. Cuevas, and L. Sternberg. 2012. An Isotopic Study of Diet and Muscles of the Green Iguana (Iguana iguana) in Puerto Rico. Journal of Herpetology 46:167-170.
Durso, A. M., and S. J. Mullin. Ontogenetic shifts in the diet of plains hog-nosed snakes (Heterodon nasicus) revealed by stable isotope analysis. Zoology.
I think this paper will be useful: https://www.researchgate.net/profile/Soren_Toft/publication/232950436_A_method_of_obtaining_dietary_data_for_slow_worms_(Anguis_fragilis)_by_means_of_nonharmful_cooling_and_results_from_a_Danish_population/links/0fcfd50add2cebc756000000.pdf
Article A method of obtaining dietary data for slow worms (Anguis fr...
Wotherspoon, Danny, and Burgin, Shelley (2016). Sex and ontogenetic dietary shift in Pogona barbata, the Australian eastern bearded dragon. Australian Journal of Zoology 64, 14–20