Used sand filter. How do I check to make sure it works?

I don't believe you will find the fittings you need at a hardware store. The 1.25" fittings are not standard size. You could use some of those rubber couplers, but that's about it.

Have a read through this thread.

Intex sand filters have a Min fill and Max fill line on the center pipe inside the sand filter. Fill sand to halfway between those two lines. Cover the top of the pipe with a cup if you didn't get the pipe cover that comes with it, so you don't get sand down that pipe. If you can find the model number on the sand filter, you can use that to find the manual online at intexcorp.com, if you don't have that either.

You won't find what you need to convert at a hardware store. You'll have to Ebay, Amazon and Walmart.com it. You will need: Amazon.com: BOS Complete SFX to Ground Pump Conversion Kit for Summer Waves Pools: Garden & Outdoor. Might want to see if you can buy each piece separately and make it cheaper. Then you'll want these if you don't have those either: Amazon.com: Intex Above Ground Plunger Valves with Gaskets & Nuts Replacement Part (2 Pack): Garden & Outdoor

Put the plunger valves Directly on the Sand filter, then put the reducers that are in the kit on the plunger valves, then your hoses. These things are ALL that you will need. You will NOT need Anything else, except that I recommend 3 worm gear hose clamps(1 at the skimmer hose connection, 1 on the reducer hose connection INTO the sand filter from the skimmer, 1 on the reducer hose connection OUT of the sand filter back to the pool) rather than the plastic ones that come in the kit. You don't need Hayward anything. You don't need PVC pipe, or ball valves...you do not need to hard plumb anything to make this work. You might end up with an extra hose(the one you took off the skimmer because it's not long enough to reach the sand filter) but keep it and the Summer Waves pump you're removing, so you have a back up way to filter in case the Intex Filter or Pump fails.