This is a good article on testing to see if one is truly sensitive to a given agent applied to the skln. There are two tests: the Repeated Open Application Test (ROAT) and the Provocative Use Test (PUT). Here's the article: Download J_002
Generally, the skin in front of the elbow is a good place to use for the ROAT. Apply a small amount of the cream/ointment/spray -- etc. a couple of times a day for a few days -- if no reaction within 4 - 5 days -- one is probably not sensitive to the agent.
Here's the abstract of the pdf above.
As one step in defining the clinical relevance of exposure to an allergen identified with patch
testing, use tests (provocative use test (PUT), and repeated open application test (ROAT)) have
been used. In 1/2 of the cases of seemingly reliable patch tests, use tests are negative, suggesting
that the patient’s biologic threshold of response had not been reached with open application dosing.
Dramatic differences exist in regional skin reactivity and percutaneous penetration. Negative
results of use tests on normal skin may become positive on diseased skin. To refine this assay
further, more controlled observations and analysis of reaction differences between normal and
damaged skin, and among regional anatomic sites might be performed. In addition, we require a
standardized measurement for the results. Use testing has significant potential in refinement of the
evidence-based diagnosis of clinical relevance. However, for general validation, we should fill the
deficiencies described above.