incomplete_input?

ec = Encoding::Converter.new("EUC-JP", "ISO-8859-1")

begin
  ec.convert("abc\xA1z")
rescue Encoding::InvalidByteSequenceError
  p $!      #=> #<Encoding::InvalidByteSequenceError: "\xA1" followed by "z" on EUC-JP>
  p $!.incomplete_input?    #=> false
end

begin
  ec.convert("abc\xA1")
  ec.finish
rescue Encoding::InvalidByteSequenceError
  p $!      #=> #<Encoding::InvalidByteSequenceError: incomplete "\xA1" on EUC-JP>
  p $!.incomplete_input?    #=> true
end