Answer: it’s very difficult, even more nowadays. In fact it depends on what you understand by ‘expensive’, as there are two ways of understanding it, either as a price or as a value. Prices are determined by variable criteria that are not necessarily lasting. For example, a painter may be paid more for his work if he has a long list of expositions in most famous places, but also if he finds a good ‘sponsor’ or ‘mecenas’. Which does not mean that the work of art is worth something. Conseuquently, after some time, the price will fall and the artist forgotten.
The value of an art work is also variable depending on different tendencies and usually appreciated by some horrible people that are called ‘art critics’. Some are useless because they use art as vehicle for political or other ideological positionings, which are relative to themselves and not to history in general. Such, for example, one of the last winners of a European art contest in London, where the winner had represented a portrait of President Bush imitating a photograph of Adolf Hitler. It is obvious that it won for sheer political reasons in an absurd attempt of criticism, which to my understanding lacks of intelligence and finesse.
If a art critic is a little bit more objective, he takes into consideration a certain number of criteria which are those that determine finally the work of art. Such as (in painting): originality of motif or composition, specificity (differentiation) in matters used or colour combination (many painters spent many years in searching almost chemical reactions that allowed to have colours of their own, as Klimt for his ‘gold’ or that allowed that the canvas and colour would last in time, oil, for example, or give a special shining to his works, as Bretagne living Italian/English painter who used terebinth oil – usually used to clean nets for fishers – to give the colour a special, vivid shining). The balance in volumes, the clarity in exposition, the difficulty in technique are also criteria taken into consideration, to which can be added the punctual representation of a historical or personal moment even in bizarre pictures, such as Goya’s ‘Uranos eating up his children’ which is technically not a master piece.
Vamue is given also by a name, or the fact of belonging to a determined artistic current. A Rembrandt is always expensive because it is a Rembrandt, or a Van Gogh, even if it is works of less quality or less original. A ‘surrealisme’ is also always expensive, even if it does not belong to a very well known master. Lately the tendency to ‘distructure’ art, has made expensive many things that are of little value because material used is not lasting, so that they will disappear. Thus, you may have bought something for many thousands that is of value none or acquire for cheap something that is very expensive.
Personally I think that art is subjective, so that it is made by the liking and not by objective criteria. Strangely though, objective criteria join fine taste, which is less common (Gertrude Stein) but determines prices in markets. Conclusion: before trusting an art critic, buy what you like, and hope you’ll find the thing.