There may be standard methods to consider if for your need to estimate or for research grade work.  I dont know if there is a best way, but there are a variety of ways that have been used, but doing this on small samples taken to a lab usually involves using equipment that takes a specific size sample, protecting from water loss, weighing sample and drying.  If you want to measure this repeatedly, there are sensors that can be installed in the soil if you wanted to perhaps test responses to wetting and drying cyles in a lab, or if you want to go mobile to field conditions, surface or sampling with depth can be done with various nuclear surface or well probes that can repeatedly be sampled to detect soil moisture and density changes with conditions.  There are various companies that make this type of equipment. 

I added a couple of papers that I saved, unsure if they will be useful or meet your demands.  I would mention that if you are trying to test for something unplanned from soil samples already collected, soil moisture may have changed or the sampling procedure and if storage not ideal.  I say this because the question suggested that the soil samples were small -- why?  Perhaps for the accuracy desired or needed, larger or insitu testing is needed.  Extremely small samples are apt to have more variability, unless the soils are extremely uniform and variables such as rainfall distribution, growing plants and rood density add to variability.  Best for site or project critical or research grade work to always know the standard methods to be used and then consider what process and procedures including sample size to be applied.

You need to define what a "small amount" is. I have attached two of my papers describing TDR probes capable of measuring dielectric permittivity and electrical conductivity in soil samples as small as a few cm3.

Article Estimating Water Content from Electrical Conductivity Measur...

Article Shaft-Mounted Time Domain Reflectometry Probe for Water Cont...