Engineering drawings come in two flavours metric and north american (mainly US only). Metric drawings are using mm as a unit, even in architecture where a mm is a slightly small unit. North american drawings are using inches as their base unit. If they would use anything else then the unit would read next in the dimension (this is incredibly rare).
Now, the sheet states that the unit is mm in the title block. The title block applies to the whole drawing. Other much weaker hints are the A4 size paper (ISO size indicates mm), projection rotation rule (image is drawn in first angle, US almost exclusively use third) and scale info. Although you really shouldn't measure distances on drawings since the numbers are what counts and there can be reproduction errors and/or dimensions not in scale.
Other possible hints would be invoking any ISO standards vs ASME standards which here are not present (for example a general tolerance). Also simply assuming mm* would work for the bulk of drawings made for any other purpose than US consumption. Besides in this case inhes would sound incredibly big.
* Although, inches are much more common in electronics than mechanical drawings, so best not guess anything.