It turns out that your question is not an easy one. Your best bet is still to try some of the leas kits that are available on the market. To really do quantitative measurements of lead researchers do atomic absorption spectroscopy or other procedures that you cannot do at home.

A friend from Geology pointed me to this website for a lead testing kit:
http://www.professionalequipment.com/lead-in-soil-sampling-kit-60447800005/lead-test/

Unfortunately it looks like it's a mail away thing, but I think that might be the best bet at measuring the actual levels.

In laboratories a common way of determining the amount of lead in some other media is to separate the element from the surrounding media and then oxidize it electrochemically to lead oxide. This, however, is a method that cannot be recommended for non-scientists to use. The lead test kits are only meant to indicate the presence of lead. Different agencies report different permissible thresholds for lead content. The highest one is 0.1 mg/m3. This is very low. For all practical purposes, if the lead test kit can detect lead, then there is too much lead in the soil. The test kit that you use should indicate the sensitivity levels.