in monetary/financial economics parlance, how is 'excess liquidity' measured?

There are several measures of excess liquidity and there's no way I can type the formulae in here, but I'm sure you can find those on the web.

One measure is called "price gap"

Another is "real money gap" which is an alternative version of the price gap

Another is the "nominal money gap"

Another is the "monetary overhang"

Excess liquidity is a fancy term to express an economic situation where people are keeping thier investments in things that are easily converted to cash. Or, to look at the opposite viewpoint, they are not investing in projects that will tie up their money for a long period of time before repaying the investment.

Generally, long term investments are required to improve infrastructure, create jobs and ultimately flow money throughout the economy.

Example:

A person can put 1 million dollars in a bank and collect interest. They have easy access to that money; it is a liquid asset.

OR

A person can invest 1 million dollars in a company. They will get a small portion of the profit until they sell their interest in the company. In the mean time, their million dollars is spent to get the company going or make improvements. The money spent goes to paying salaries and providing materials to conduct the business. This money churns through the economy.

Excess liquidity is one of those terms that sounds like a better deal than the reality it represents.