I always take parameter readings before I make the slurry. Otherwise, you have no idea where you started from.
I'm happy to outline my method, which I didn't do before.
Start with cleanest water, I use RO/DI, but in this instance distilled could be a very good way to go. Measure EC and pH.
Add enough of your media samples to make a slurry.
You obviously don't want it too thick, or it's going to be difficult to get a reading. I want it to be like... like crepe batter, I suppose, you want a very loose consistency.
Stir well, and then allow to sit for 7-10mins. I usually end up testing at 7mins, because I've done comparisons between 5mins, 7mins, 10mins, 15, 20, and 30. After 10mins I observe no shift in values.
Now, you can either strain it to test, or just dunk your meter in there and take your readings. What you're after is whether or not anything has shifted. Obviously, EC will likely be the biggest parameter to change, but, for example, when growing in coir, I want to see that pH shift from around 6.5+ (where it lands in my storage tub) to the high 5's. If I don't see that shift, I know the root zone pH is out of whack.
When I've tried to just test run-off, I get pretty crazy readings and have ended up chasing my tail. HOWEVER! I know that many people are better able to resolve their issues by using run-off pH, so I don't tell folks they *must* do the slurry test, just what I prefer and why. :)
I hope that answers your questions.