I don't think you can determine founder without x-rays as founder is the rotaion and/or sinking of the pedal bone (perhaps you can tell in a bad case??)
If the pedal bone has rotated or dropped, it is important that steps are taken to prevent furter rotation or sinking. If the bone is left to its own devices, it can rapidly get worse.
Someone mentioned riding a foundered horse - I assume they mean a laminitic horse as exercise can cause further deterioration until the bone stabilises.
I hope your horse make a fast recovery.
ETA: I don't mean to worry you as horses can recover, but founder can be life threatening and so must be taken seriously and time is crucial.