Yes you can. There are TDS pens, bout 2-3 time thicker than a pen, alot of LFS carry for $25-35. Super easy, click on, place in water, read numbers and turn off
A lfs sold me rodi with like 250tds... I was pretty pissed...all it made me do was set up my rodi again... the water machines out here are around 20tds
you never know for sure when the service dates on those have occured. if just serviced it may be ok. If it is 5 month old filters etc inside then not so much.
These things are great and you cant beat the price.