How do you know the thermostat is bad on a 1998 Chevy Venture Van

it will overheat or it will not heat up to op. temp/ gauge stays low.
book says remove it and put in pan of water with temp gauge and boil it, see what temp water is at when thermostat opens.
i'd say just replace it.
what kind of problem are you having?

It's recommended to clean and flush the cooling system every 2 years and further to repalce the thermostat during every flush. How long has it been since the last flush?

If you've checked the rest of the cooling system and still suspect the thermostat, just replace it.

Reason being, its kinda difficult to get at and the recommended method for checking it to remove it, put in in a beaker full of water and heat that water to the temperature indicated on the bottom of the thermostat amd see if it opens at that temp. If not, replace it. If so, it's good
The thermostat is not very expensive for these vehicles, I have one myself and have changed the theromstat twict over the years.