I am trying to design a cad model that snaps onto the sides of the eye things on the gas mask, but I don't know how to measure it in order to represent it geometrically.

I would first recommend you search for a CAD model online for the same product or search for its standard dimension. if you didn't find any, you could use many tools to measure it accurately, for example by using a digital caliper, or for greater details you can use a digital microscope for more accurate dimensions( but I don't think this case requires this accuracy), so I greatly recommend using a digital caliper and measure the key dimensions that mainly effect you design purpose. Remember a good product requires a try and error procedure. 

good luck traysonc! 

Hey traysonc,
If you want to create a snap fit, first get a CAD model of the product. If not, try to project the surface using some tracing method to get a rough estimation of the dimensions. if you need help on any CAD modelling for the product, please let me know.

a 3D scanner would be more accurate for this.  
Otherwise, purchase some cheap radius gauges from Amazon, get some calipers, and do it the old fashioned way. Use the gauges to measure the curves