When you look at a drawing how do you know if you are looking at inch or millimeter valueswhy is ti important for an engineer to know this piece of information?

if a drawing is in inches it does not have leading zeros on the units ex .162 instead of 0.162

if a drawing is in millimeters the dimensions will have leading Zeros ex 0.875

it is important to know which units you are using otherwise you could build a component the wrong dimensions say a tube that's 1mm long and a 5mm diameter instead of a 1in length and a 5 inch diameter

also, most engineering drawings will specify the unit in the title block to eliminate confusion