Interesting puzzle...I dunno, but let's see what we can find...
The class is the fully-qualified distribution name; in your above example 'prob.NormalDistribution'; not just the parent class.
Well, let's see -- some testing shows that mustBeA is case sensitive as well and must be eggzackly the matching name -- so, the following seems to work with the builtin distributions -- fortunately, makedist without arguments returns the list of allowed/known distributions.
>> plist=strcat('prob.',capitalize(makedist),'Distribution')
{'prob.BetaDistribution' }
{'prob.BinomialDistribution' }
{'prob.BirnbaumsaundersDistribution' }
{'prob.BurrDistribution' }
{'prob.ExponentialDistribution' }
{'prob.ExtremevalueDistribution' }
{'prob.GammaDistribution' }
{'prob.GeneralizedextremevalueDistribution'}
{'prob.GeneralizedparetoDistribution' }
{'prob.HalfnormalDistribution' }
{'prob.InversegaussianDistribution' }
{'prob.LogisticDistribution' }
{'prob.LoglogisticDistribution' }
{'prob.LognormalDistribution' }
{'prob.MultinomialDistribution' }
{'prob.NakagamiDistribution' }
{'prob.NegativebinomialDistribution' }
{'prob.NormalDistribution' }
{'prob.PiecewiselinearDistribution' }
{'prob.PoissonDistribution' }
{'prob.RayleighDistribution' }
{'prob.RicianDistribution' }
{'prob.StableDistribution' }
{'prob.TlocationscaleDistribution' }
{'prob.TriangularDistribution' }
{'prob.UniformDistribution' }
{'prob.WeibullDistribution' }
The above serves to make sure only it is one of the possible; you'd have to shorten the list to specific values to find out about a particular distribution in which case it's probably still just a simple as to do a string search.
I dunno what you do about added distributions; didn't 'spearmint with one of them...
