There are no mile markers in Google Maps and no way to toggle them in the view. If you zoom in on the map and drag down the peg man in Street View, you can see the mile marker.
If you need to see the mile marker, look to the exit number near where you are trying to go. Exit numbers are often associated with the miles where they are located. Find the exit number closest to the mile you are seeking, then zoom into the area, and click on Street View. From there, with your mouse, pull down the peg man that lets you look head-on at a location, At this point, you need to adjust where you are viewing by clicking the arrows that appear in Street View.
Clicking on the arrows moves your view forward or backward depending on the arrow you choose. Click on the arrows, zooming in on the mile marker signs as you pass them. For most roads, as you head north, the mile numbers get higher, and as you head south, the numbers go lower. Once you find your first mile marker, proceed north or south depending on the mile number you are trying to find.