Run validations when submitting a Proposal

Summary

Our customer would like to run validations (e.g. mandatory check) when a Proposal gets submitted for approval.

Content

Currently we use a handful of simple groovy scripts to run validations that are triggered when saving the Proposal (Before update triggers) and these work ok-

But from a business perspective the optimal point in time to perform some of these checks would be when the Proposal gets submitted for approval.

Any advice on how that could be accomplished?

Thank you in advance,

     Carsten