How do you know if you are running lean???

[slsail]

Is there a way to tell?

[boats.net]

A lean condition can affect any or all ranges, poor idle, hesitation at mid range or loss of top speed. It can be difficult to tell what is making a engine run bad. What kind of problem are you having?

[Operaman]

Some signs are: Rough/Coughing/Stalling at Idle.
You lean on the throttle and you get nothing for a few seconds then the boat launches is a sign for mid-range as well as rough engine at mid-range.
High RPM and the engine slows down, runs rough or never gets the RPM's up.
Hope that helps. I am sure there are guys here who have more.
Why are you asking?

[hkps1]

Without knowing what the issue you are having diagnostics are out of the question.
But one sure way to tell if you have a lean condition is to do a plug reading on all the cylinders. You can do these at different RPM and see just what is going on with the fuel ratio.

[blazerblobs]

In a proper running engine, the spark plugs look like old chocolate that has been sitting out for 2 months. Wet or really dark shows rich and chalky or really whitish in color shows Lean. Just to get a quick idea of what is going on.
Good Luck