It really depends on the client. If the client is obese I may use the wall as a first line of assessing and work from there simply to alleviate any issues of getting up and down. If not, I would have them perform a full pushup with a small incline like a bench or windowsill (or Smith machine bar as Harris mentioned) to alleviate any knee issues. It will make the exercise easier than a full pushup on the toes–because of the incline–but more difficult because it’s more body weight than being on the knees.
Another option is to simply start with some lighter free weights or body bar and perhaps a chest press to assess strength and form– and work up from there.