Yes, pseudouridine.
Pseudouridine is a nucleotide, not mRNA on its own, but is used in the spike protein mRNA of the vaccine as a substitute for uridine.
Presence of pseudo uridine would indicate presence of the mRNA, if in an RNA polymer.
But the presence of vaccine mRNA is determined by the sequence, not the presence of pseudo uridine.
Pseudouridine was used to in theory decrease the inflammatory response of the body to singe stranded RNA. This response is what most likely causes pericarditis associated with the vaccination.
Thanks for the 411. My understanding of the nucleotides is rather sophomoric as is from undergrad genetics and cell biology from 25 years ago.