If a solid lump floats, and it burns with a waxy smell, it is a polyolifin.
If you can permanantly distort it at room temperature by applying a severe load, then when you heat it to it's softening point, it springs back to it's original shape, it is almost certainly XLDPE.
Also, when it melts, it doesnt turn into a liquid, but becomes a soft rubbery compound that cannot be moulded.
If you wish to quatify the level of cross linking then you need to perform a gel contnet test. This basically involves boiling a sample of the polymer in xylene and see what percentage of the material remains undissolved
