Unfortunately, you may not be able to tell if they work until they are installed and under pressure. Depending on the TPMS sensor, they don't do anything until they see pressure more than a standard atmosphere and / or movement that acts like a spinning tire. That is in addition to needing sensors that are Mercedes specific for your year and model, and the fact that some aftermarket sensors that claim to work just don't. The reason they last a certain number of miles is that they only transmit when the wheels are spinning. The batteries also discharge over the years.How many miles are on your Sprinter? The TPMS send units tend to last about 100k. If you are close to that or over then maybe replace them. On the other hand your van will drive just fine when a send unit dies -- it's not like you will go into limp mode.
Based on what I have read here, many tire installation places screw up the Sprinter TPMS selection and/or programming. Best to go buy them yourself after verifying what exactly you need. That ends up being cheaper too. You need 315 Mhz units made for Mercedes Benz. You don't want 433 MHz units or universal units. Not all 315 Mhz units will work.
Available all over the place at very different prices. But few places will make choosing the correct units easy. I would trust this outfit for selection but their prices while pretty good can be beat: https://tpms247.com