Force a role to insert a value for a custom taxonomy in a custom post

Hello everybody,

I have a custom post, with a custom taxonomy.
When the editor insert a new post I need they to chose a category ( custom taxonomy ).

I read around and I found a conceptual street, but then need some advice to make it real!

I use the hook “save_post”.
I check the role, then I check in the $_post variable if received the value of the taxonomy, and if not, I send them back to the page with a message “hey, don’t forget to insert bla bla bla…”.
First problem: how to make the checks only in the page of my custom post?
Second problem: how to send them back with the allert message?

It make sense… eheheh… only… now the syntax… mmm…

thanks!