You test for TDS with a TDS meter. Either a run-through style, which is what I use, or a handheld type that you swirl in the water for a few seconds.
Either one is good.
Don't assume your RO/DI unit is producing pure, clean water. Filters get dirty, water can be soooo nasty that you might need several filters, etc. etc.
fwiw, when I lived in Florida our tap water came out of the faucet around 700ppm TDS! That's NASTY! I had to use 2 sediment filters, one carbon, one RO and 2 DI filters to get 0 TDS. Never had any algae issues either. WORTH THE PRICE!