Measuring the Concentration of GHB
What a lot of people do to test the concentration of their gear is take 1-2ml to start off with, then wait 1.5-2 hours to see what happens. If they don't reach the desired effects, they take another dose 1ml higher than their last, wait 1.5-2 hours... And you get the idea.
If you've got 1,4-B, watch out because it doesn't mix entirely with water, and tends to sink to the bottom of the bottle if left to sit for a while. So when you're testing for concentration, you might find that the stuff at the top of the bottle is quite weak and assume the whole batch is... But as you near the bottom of the bottle, it will get stonger and stronger. So it's a good idea to give it a shake before you do anything with it.