  We present a simple method for determination of absolute vibrational
numbering of isolated near dissociation levels in diatomic molecules, usually
observed in cold physics experiments. The method is based on the isotope shift
and works even when energies of only two levels from one isotopologue and one
level from another isotopologue have been measured. It is demonstrated on data
from recently reported precise measurements of binding energies of levels lying
close to the dissociation limits in ultracold Yb2, CsYb, RbSr and RbYb
molecules. Its predictions agree with these of much more elaborate
multi-isotope potential curve fitting.
  PACS numbers: 31.50.Bc, 33.20.Kf, 33.20.Vq, 33.50.Dq
