There are a lot of methods or techniques to detect aggregation or protein misfolding. So, you can use a variety of methods to detect A-beta: You may use immunohistochemistry (IHC), radio-immunoassay/ELISA... assays to detect cellular A-beta.
Article Consistent immunohistochemical detection of intracellular ??...
You can also verify your findings through neuroimaging techniques such as PET or analysis of standardized uptake value ratios (SUVRs) for A-beta deposition:
Article Estimation of amyloid distribution by [18F]flutemetamol PET ...