I understand you should take it when the oil is hot right after a good run. And take the sample after the oil has been flowing for a few seconds - if taking it from the drain plug. If you are drawing it via an extractor through the dipstick tube, you should draw the sample from a level about half way between the full and empty on the dipstick... you don't want to be taking the sample from the bottom of the oil sump.
I suppose that if you take it from the drain plug but don't want to do a full oil change, you can capture the drain oil in a clean catch pan and simply pour it back into the engine after filtering it through some cheesecloth or a fine mesh filter.