Ok this is what i have used in my case with the advices of Evolena:
<$edit-text tiddler="Temp Tiddler" placeholder="Title" field="draft.title" tag="input" type="text"/>
This is a input where i can enter any title for a new tiddler, then aside of it:
<$set name="myVariable" filter="[{Temp Tiddler!!draft.title}is[tiddler]]" value="exist" emptyValue="">
<$text text=<<myVariable>>/>
</$set>
which allow me to display a message if the name is already taken (i was having trouble to find the right number of brackets and braces in the filter attribute)
Due to Tobias a PR is pending to correct this for the next tw version (5.1.10)
Thank you both for your help! Btw Tobias no big deal but it's a "L" at the end of "Spangenhelm" not a "i" ;-)