The easy way to tell if you have removed all the finish is to look at the surface in a reflected light. Remaining finish will show up as shiny areas, in contrast to the dull areas where the finish has been removed.
Don’t be fooled by stain still in the wood. Rarely does stripping remove all the stain. As long as the finish has been removed, however, you can stain right over the remaining stain and get an even coloring as long as the color remaining in the wood is fairly even.
The one thing you can’t do by applying more stain is make the color lighter, only darker.