When using colour negative film you would place the incident light meter in the shadows and obtain a correct rendition of the shadows. Due to the very large exposure latitude of negative film you would very likely get most highlights properly in any case. Just meter for the shadows when using colour negatives. Set your light meter to the nominal film speed, and meter for the shadows, that is.
When using B&W roll film, do as above with colour negative film.
When using slide film in a high-contrast situation where the highlights must (obviously) not block or be overexposed and the shadow detail is important as well, you can either use the incident light meter in both lighting situations and see how far "apart" they are to try to estimate what the final image will look like. You will meter in any case for the highlights as indicated by the instrument, the second reading will only give you and idea of where the shadows will block. In these situations a negative film would typically perform better due to its much broader exposure latitude.
As an alternative in the above case (slide film, high contrast) you would use a spot reflected light meter, "placing" the highest light on top of the characteristic curve of your slide film (let's say 2.5 EV above middle grey) and check, with the instrument, what happens to the shadow areas and where will they block.
This is not so easy as it sounds though because, being a reflected light meter, for each measure you have to notice if the reflectivity of the subject, on the measured spot, is way apart from "middle grey".