In order to identify the handwriting on the paper, the expert would need
a magnifying glass at the scene
a control handwriting sample from those involved
a dna analysis
a chain of custody document

a control handwriting sample from those involved

Explanation:

When experts need to identify the handwriting in a paper, they use sample documents from people involved to make comparisons and determine who is the person that wrote that. This process of analyzing the handwriting involves checking the documents for differences that can help to determine if the same person wrote the two papers and if there are enough differences, experts would conclude that both papers weren't written by the same person. So, according to this, the answer is that in order to identify the handwriting on the paper, the expert would need a control handwriting sample from those involved.