Electric installation - find connected corrugated pipes

Question

We are moving to a new flat. This flat has many empty sockets in the walls. These sockets are connected using empty corrugated pipes, so one can put additional cables through them.

I want to use these empty sockets/pipes to put CAT 7a cables into the wall, with CAT 6a outlets in every room.

My question is: How do I find out from where to where those empty pipes go?

I already tried two things:

1. I tried to push a cable through them, and check where it comes out of the wall. This worked for some connections, but for many the cable gets stuck and I cannot find the other end of the pipe.
2. I also tried to put a thread with a bit of paper on it into the pipes, and suck them out somewhere else with a vacuum cleaner. This also worked for some connections, but I need to check every other pipe end in the flat to find the correct one. And it makes a lot of noise (the pipes resonate).

I thought of pumping some sort of smoke into a pipe using a compressor, and look where it comes out, but I have neither a compressor nor means to produce smoke in a safe way.

Any other ideas?

Edit: Here is a picture of one of the sockets, with the CAT 7a cables already put through them:

Answer 1

The basic tool for installing wiring in conduit is a fish tape

It's nothing more than a stiff strand of steel, plastic, or fiberglass made to push into conduits. You push it through the conduit, tie your wires to the end of it, and pull it back through.

When you're not sure where the conduit goes, you will want a non-conductive fish tape - you don't know if it might come in contact with live wire, in an electrical panel, etc.