What jaguar said, but mot people who fill at home just do visual inspections, if that. Thankfully the tanks are overly built. The tank "should" be rated to 1800psi, and normally CO2 never sees much past 1000 so it's not like you're stressing it, but if over filled it is possible. Google a CO2 pressure/temp chart at normal filling level to see what to expect in your environment. So I look for cracks and take a look-c inside for corrosion. Also, "if" you carry the filled tank in your car and it's expired, it's a serious penalty. I think... Been a long time since I read the law but I kinda remember it being equivalent to murder or something redonkulous. Like carrying a refilled 1lb propane tank is like 5yrs prison, which is imo a direct violation of the 8th amendment. Does that mean if you had 10 of them for a camping trip it's a life sentence?
Now I'm not saying to skip inspection, just saying that many/most people do. It's probably cheaper to buy a new one than hydro it. My last hydro was for a bigger 88CF 4500 tank but I remember it being ~$20, and my 20oz CO2 tank was ~$12.
Makes me wonder if a tank hydro is only good for 5yrs then what about the gun? The tank is certainly safer than the gun yet I've never heard of anyone testing a gun. Not that I'd worry about CO2 as a general rule, unless over filled, which people do, but I really wonder about 3K+ pcp. People usually charge those with moist air so I rekon they rust and some/most people never open them for a visual.
So have you acquired the fittings and such to fill yet? My fill setup was simply the two needed fittings for the two tanks, each connected to a T-fitting via short lengths of 1/4 pipe, and off the side of the T was a small generic 1/4 turn hand valve to vent pressure so I could fully fill the receiving tank. The receiver needs to be vented of its gas to fully fill with liquid, if that makes sense. Then once completely full I disconnect it, tilt it, then vent liquid to get a reasonable safe level. Plus it's nice to vent before removing the fittings. To me the gun is easier, you either don't vent it which will give you and under fill, or you fill 100% then dry fire xx shots to vent the appropriate qty.
There is also the option of a steady feed from the big tank via a long flex line, which I suppose would be cool if shooting bench. Pressure would be more consistent too. At work I have some very thin very flexible plastic tube, like .085" OD and .040 ID that I have taken past 500psi and it seems fine. If not some -3AN teflon/stainless braid brake line would work, and shouldn't be too much $.