I want to send my gf flowers on valentine's day under her ex-boyfriend's name
Why on earth would you do that?
I want to find out what her reaction is
This is tricking her to see if she still have feeling of her ex. That's wrong and it shows you don't fully trust her. Just break things off if you are going this far.
She doesn't deserve my trust ;-)
You don't seem too trust worthy either. Like what do you expect her to do tell you and possibly make you jealous? Throw them away?