I would like to have a numeric field and limesurvey automatically places the € symbol in front of the entered value.
It's possible?

So, you want to have the currency symbol stored in your data?
Or just have displayed in front of the entry field?
Second is possible with "suffix" rresp. "prefix".

Joffm

And how can LS select the correct currency symbol?

just have displayed in front of the entry field
Just put in the prefix the € symbol?

Try it.