In his comment (and on his excellent blog, Social Democracy for the 21st Century) LK argues that it's impossible to define, much less measure, socially necessary abstract labor time (SNALT).
First, the "abstract" in SNALT refers to labor abstracted from the specific things that laborers do. Marx argues that because the specific tasks are incommensurable (how do compare sewing a seam to gluing a sole to a shoe?) the specific tasks used to create different commodities cannot be a basis of a consistent exchange value. In this sense, abstract labor time is just the fact that someone has worked for a specific period of time, without regard to the specific tasks that person has performed.
But there are definitely factors other than time that affect productivity. LK asks, "How do you take an average of a heterogeneous factor like labour, when there is so much difference in profession, skill, competence, experience, and skills to be 'averaged'?"
To get average abstract labor time per unit, first count the number of objects produced, count the total number of raw person-hours used to create those products, including the time necessary to create all the capital, all the skill training, and all externalities, and divide by the number of objects. If you want marginal SNALT, find the least efficient producer who is still in business, and do the same thing just for the last unit they produce. Standard economic theory predicts that the marginal cost of the last unit should equal the minimum average total cost and the price. (LK challenges this aspect of standard economic theory, and it might be incorrect, but marginal cost is not exactly a novel economic concept.) Any effects other than time, skill, competence, experience, etc. should all be normally distributed and should cancel out in the aggregate. This is not rocket science.
Of course, the Labor Theory of Value (LTV) is not the only thing affecting actual money prices. You would want to look for shocks (the price of gas just after the beginning of the Iraq war, for example, would probably not reflect SNALT), monopoly and monopolistic competition (SNALT is a valid predictor of prices only under perfect competition), hidden positive and negative externalities, imperfect or asymmetric information, network effects, etc.
Remember, Marx never intends the LTV to be a tool for predicting prices that hedge funds can use to make a lot of money in arbitrage. The LTV is a conceptual tool to explain what it means to say that the capitalist exploits the working class: the capitalist class expropriates labor time without compensation from the working class.