Compute the mean of a data blob during testing phase

I am using hdf5 layer that has data and label.

layer {
  name: "data"
  type: "HDF5Data"
  top: "data"
  top: "label"
  include {
    phase: TEST
  }
  hdf5_data_param {
    source: "./list.txt"
    batch_size: 8
    shuffle: true
  }
}

During the testing phase, it will load 8 images from the testing set and feed the network. I would like to print the mean value of each image during the testing phase. Is it possible to use it in CAFFE? Which layer should I used?