I have a requirement on Loading Standard Opportunity Detail Page, if Opportunity Amount is greater than certain value say 500K i need to display a message on top of page. is it possible without performing any DML operations on the page to get the message. I thought of all possible ways but could not figure it out.
The simplest way would be to use a formula field located near the top of the page. Perhaps an image formula, so you could display caution symbol etc.
More info on it here:
This method is a workaround and maybe not officially supported. But you can mimic the look&feel, placement and behavior of the message exactly as you require and you will have greater flexibility.
If the look&feel is not crucial you should prefer the method suggested by @DavinC