There you'll find general resources on Symfony 1.x development (in the Community Development Recommendations page), Entity Relationship Diagrams of the database, tips for contributing, information about our code repository, and more.
In terms of the code, I would suggest that you study one of the standards based plugins to see how we have implemented things.
For example, in the ISAD plugin, when the view page is rendered, you can see that a whole bunch of of the mandatory fields are passed to a $validatorSchema variable, to be checked whether they should issue warnings or not when the page loads, starting at line 78 down:
It looks like the errors that are found by this action are shown using this slot on the view page:
Meanwhile, the edit page template has a set of elements used to indicate required fields, and set the warning if they are not populated, as seen on the title, here on line 51, and elsewhere in the same file on other elements:
And the edit actions appear to have some kind of field validator function in place, as in this line example:
I would recommend that you keep exploring one of the standards-based plugins to see how the code is organized and used, so you can trace how mandatory fields are indicated and passed to validation actions so a warning can be returned if the criteria for success is not met.
If you have a specific question about this that a developer might be able to answer, let me know and I will see if I can get some further input for you.