Using Events action on sales quotation to validate price list

I am trying to add a Valid To customer Field in Sales Price List. The standard field ‘Valid From’ and customer field ‘Valid to’ will be used to validate a sales price list. If the validity is expired or invalid the event will trigger on sales quotation line and prompt an error that the sales price list is not valid. 

I have successfully created the custom field. 

The event works on LU which is selected , in this case it will be Sales Quotation but the question is how will i compare ‘Valid from and Valid to’ from sales price list and prompt an error.