It totally depends on WHY the horse was fired. If it was a major unsoundness, it is likely to come back with hard riding. If it was minor and given time to completely heal, it is not a big deal -- just a 'blemish'.
The only way to find out would be to track him to his original owner which would be difficult to do without his papers.
A PPE would help, but a comprehensive one will cost a lot and have to include x-rays. And even with one, the history is a lot more important, at least for me.