Question

Is it possible to evaluate two factors, to see whether they have the same numer of levels and if so which levels are not present in one or the other?

Answer 1

If factor1 and factor2 are your two factors, just look at levels(factor1) and levels(factor2).

Same number of levels:

length(levels(factor1)) == length(levels(factor2))

Values in one and not the other:

setdiff(levels(factor1), levels(factor2))
setdiff(levels(factor2), levels(factor1))