You can look into the metadata, (LED or TRL file, I dont quite remember) if you wanna know the acquisition mode, |
and the other way, you can look in its file/folder name, it is mention the acquisition mode in the filename,
ENVI is only support Level 1B1, 1B2R and 1B2G of ALOS PRISM, if your data is level 1A, I suggest you to use ERDAS IMAGINE which it is fully support ALOS PRISM, include the DEM extraction