There are two aspects to this question -

Number 1: How to determine the cargo quantity which can be carried in a particular hold?
Number 2: After the cargo has been loaded, how to determine the cargo quantity loaded in that hold?

Number 1: The maximum cargo quantity which can be carried in a particular hold can be estimated by using a simple mathematical formula i.e. mass = volume x density or mass = volume / Stowage factor (as stowage factor is the reciprocal of density).
This will give you the mass of the cargo which can be loaded in the hold. HOWEVER, this may not be correct as the total mass of cargo which can be loaded in a cargo hold is limited by the fact load density which depends upon the tank top strength in a cargo hold. In order to obtain the maximum cargo which can be loaded safely into a given hold, the vessel's trim and stability booklet (or capacity plan) shall be consulted.

Number 2 : In order to determine the quantity of cargo in a particular hold after the cargo has been loaded in to the vessel is a very vague concept and unfortunately it cannot be obtained very accurately.
The rough estimation could be first you determine the height of the cargo in the hold and using the length and breadth dimensions of that hold (given in the general arrangement plan) and determine the volume of the cargo. Thereafter, use the formula mentioned above to arrive at the mass of the cargo loaded.
Please note above works only in the case of bulk cargo.