Yes Rory
There are 2 ways
(1) Client Side Validation, you can Write javascript OnSave Entity to Validate your records. you can write your logic in Javascript which will validate user data
(2) you can write Plugin in C# or VB and then register plugin on Pre image so it will execute before records get saved. in your validation logic you can throw message or you can cancel the Save event.
Many Thanks -Bhautik Desai xRM Technologies