Summary: Fill your script with an invisible payload that fills any buffers, and put something time consuming (say `sleep 5`) early in your script in order to detect that the script is being executed directly rather than just stored to disk. If the client halts before having read all data, it is likely a `curl | bash` scenario. If it just keeps reading, it's a regular browser just downloading.
To combat this sort of thing, @jbenet made hashpipe: https://jbenet.github.io/hashpipe