What parameters?

Assuming you want any of the normal values:

To measure opacity, shine a light (at the correct frequency) across the flue.

To measure excess oxygen, place an extraction probe in the flow and run that to an oxygen sensor.

To measure unburned hydrocarbons, place an extraction probe in the flow and run that to an appropriate sensor (CH₄, for example).

To measure flow profiles, stick a pitot tube into the flow and slowly move it across.

To measure flyash, you may as well wait for Godot.

Alternatively, why don't you just contact your local flue gas sales rep? Yokogawa, Ametek, Siemens, etc. all make a living providing exactly what you want.