did the trick.
How about checking with python code:
from tensorflow.python.platform import build_info as tf_build_info
print(tf_build_info.cudnn_version_number)
# 7 in v1.10.0
cat /usr/include/cudnn_version.h | grep CUDNN_MAJOR -A 2
did the trick.
On Ubuntu 20.04LTS:
cat /usr/local/cuda/include/cudnn_version.h | grep CUDNN_MAJOR
returned the expected results