There are several methods you can use to detect intracellular ATP levels, some of which are: 

1. Bioluminescence assay - Bioluminescence assay is based on the firefly luciferase enzymatic reaction, which generates photons of light using ATP from viable cells. A luminometer is used to detect the intensity of the luminescent signal. A higher luminescent signal indicates higher ATP levels.
2. Fluorescence assay - This method utilizes a fluorescent dye, such as quinacrine or lucifer yellow, which binds to ATP and emits fluorescence. A fluorescence microscope or a plate reader is used to measure the fluorescence signal. 
3. High-performance liquid chromatography (HPLC) - In this method, ATP is extracted from cells and tissues and separated from other components using HPLC. The amount of ATP is then quantified using UV detection or fluorescence.