Finding confidence level of miRNA disease associations
You can start off by studying how an miRNA regulates the functioning of a gene. it basically binds to the 3'UTR and 5'UTR of an mRNA and restricts its translation. There will be genes associated with the disease of your interest,  find out the miRNA's related to those genes. You can use micro array data to find out the up-regulated genes. Target scan human, miRDB, are various databases for retrieving the miRNA's related with the genes. 
The confidence level for finding out the association of certain miRNA's with the disease can never be 100%. but a relationship can be established by simply viewing the complementarity between the miRNA and the mRNA expressed during the diseased state.