How to check if SSRS Textbox is empty

Try the following:

=IIF(Len(ReportItems!txtCountVolunteer.Value) <= 0, true, false) 

You should use this expression

=IIF(IsNothing(Fields!UserEmail.Value) OR Fields!UserEmail.Value = "",
 "Empty", "Not Empty")

The first: IsNothing(Fields!UserEmail.Value) checks if the field value is NULL The second: Fields!UserEmail.Value = "" checks of the filed value is blank ""

So you need both of them in order to check if the value is either null or empty.

Check for null

=IIF(IsNothing(ReportItems!txtCountVolunteer.Value),true, false)