There are many devices out there on amazon, since formaldehyde inside your house == bad is a chinese health meme where air meters will measure it specifically.
https://www.amazon.com/Air-Pollution-Formaldehyde-Detector-T...
I am not aware of a consumer sensor that measures this accurately enough, but I'm also curious what is available.
I tested my home using a badge from this company: https://acsbadge.com/ They are not cheap ($60 for one test), but it's accurate to 2 ppb and will give you an idea what your chronic home exposure is and if changes are needed.
I've ordered an awair, I think it does vocs but not sure it does them in any specificity.
And I think the only real solution is venting. Open a window. Other than identifying the source and removing it of course. But in many places I think it's wall materials, furniture, etc
Dhh had a talk on indoor air quality you may find useful.