Holmes was checking the water displacement of the putative "Stanley Cup" to determine its density. This technique is famously attributed to Archimedes who had to determine the gold content of a complexly shaped crown. This only works if there are no enclosed spaces (hollow parts).
When the prison guard mentions throwing Nikki Moreno "in the SHU" (Special Housing Unit) then follows it up with, "Uhh, that's solitary [confinement]" Joan replies with "We get Neflix", a reference to Orange Is the New Black (2013), a series set in a prison where "the SHU" is mentioned often.
Sherlock is likened to a magnet by Bell, who then says he is not a fork. This is a reference to the 1960's Isreali mystic, Uri Geller, who claimed he could bend a fork with his mind. It was eventually revealed it was done with magnets concealed in his hands.
Susan Misner who plays Loretta Nichols previously guest starred with John Noble who portrays Moreland Holmes on the J.J. Abrams series Fringe