accidentally spilled liquid on my 2020 MacBook Air. Can it be opened to check for moisture ?

Well, the pure straight forward answer is yes. However, this is generally a bad idea unless you're very experienced at doing so. You'd very likely not be able to tell anything anyway in the way of damage or future corrosion issues.

You also have voiding the warranty issue too if still under warranty.

tbirdvet is correct; have Apple evaluate it.

Only by an authorized Apple technician.