First, if you do not know about the flow inside the pipe always consider there is great possibility of fluid inside the pipe. Meaning Pipe is live. This is "Safety at First" approach.
In order to determine the flow inside the pipe I will use the following steps: (Note they are not in particular order).
1. Try to use human senses (safely)
2. Try to read nearby instrumentation on the pipe like PI, TI etc.
3. Look at the upstream and downstream of the pipe, there must be other clues to tell what is happening inside the pipe. It could be pipe connected with Pumps, Vessels, Reactor, and Heat Exchanger and if they are in operation then it is almost certain pipe is live and have fluid inside.
4. If nothing above works then I shall try to check (safely) the temperature and pressure inside the pipe with the help of modern gauges, considering pipe is live.
Hope this works.
Abdul Jabbar
Piping and Pipeline Consultant
New Zealand