How to identify whether something is in linkage disequilibrium?
As per the definition you gave, you check whether there is a statistical association between the two. If knowing when allele is present at a locus does not affect the probability of finding a given allele at the other locus, then they are independent (there is no association). Otherwise, there is an association and hence the two loci are in linkage disequilirium.

See wikipedia > Linakge disequilibrium for more info.