The plans discuss fuel tank installation but not how to sample/test the fuel itself.

Do you have a fuel sump installed, to test fuel left in the tank since the last flight?  Where'd you get the sump?

If you don't need a fuel sump because you drain the fuel tank after every flight, please post a photo of your fuel drain set-up.

Quoted from nathan.bissonette

If you don't need a fuel sump because you drain the fuel tank after every flight.....

Ok,  
Why would you drain the fuel after every flight,  with or without a sump? Pre-mixed fuel does not go bad that fast: especially non-ethanol, which you really should use if available.
While modern gasoline are much less "shelf-stable" than the gasoline from 40-50 years ago,  it doesn't go bad for several months, even without a stabilizer additive(Sta-Bil, Sea Foam, etc).
Plastic fuel tanks do not create condensation nearly as bad as metal ones.  If you are concerned about water in the gasoline when you purchase it from the store,  I suggest you buy a Mr Funnel. [url] https://www.amazon.com/Mr-Funn.....p;keywords=mr+funnel[/url]
I have one, have tested it, and it works as advertised. Also,  topping off the tank after flying leaves less room for air, so less chance for condensation.

You can also install a fuel/water separating filter on your bird.  I have a Polaris PWC filter installed on my bird's fuel line between the tank and the pump.  It is see-thru, so any water is easy to spot. You just remove the locking ring and bowl to dump the water/contaminates.

https://www.ebay.com/itm/Polar.....?hash=item486bf42b25

The Mr. Funnel is a brilliant piece of kit and I consider possession of one to be just as important as having enough fuel in the tank before flight.

In the UK the PFA/LAA who were the airworthiness people for UK Minimaxs said a gascolator must be included in the fuel system.

https://duckduckgo.com/?q=gasc.....w.alisport.com%2Fwp-
content%2Fuploads%2F2015%2F09%2Fgascolator.jpg

It is a 'sump'. If it has been installed correctly the idea is that it will collect any water and any other crap in the fuel because it has a filter also. It allows a sample of fuel (plus any water and/or crap) to be taken from the lowest part of the fuel piping system - which is likely where water/crap will sit.

Following along the lines that Antoni said, actually its more like a need to test fuel (for contaminants) before every flight (debris and water/condensation).  I have always included a pogo drain in my main tanks and also use a gascolator as another stop gap (at the lowest point in the fuel system).  Your only removing a few CC's worth of fuel (typically), and if clean, can dump it back into your tank.  If not clean, keep pogo-ing until it is (or drain the tank fully and start fresh).  Also, when piping the system be sure to avoid any low spots in the line (U's) which can hold crap in a location - until it doesn't at the most inopportune time).

https://www.aircraftspruce.com/pages/ep/fuelvalve_0drain/safaircav110.php

https://www.aircraftspruce.com/catalog/appages/acsgascolator.php

https://www.aircraftspruce.com/pages/ps/fueltesters/05-03386.php

First, thanks for the links to the parts.  More affordable than I thought.

Second, could you post a photo of your fuel sump/gascolator installation?  I assume it's in the engine compartment, forward of the fuel tank and pedals but aft of the engine?   What's it connected to, to keep it from rattling around?

Usually connected (bolted) to the firewall (as low as possible).  Many options to affix.  One below (bracket).  Some gascolators already have these affixed.

https://www.aircraftspruce.com/catalog/appages/acsgascinst.php