I've found ISTPs often give off a hard-travelling kind of image, especially when they're younger. They are usually very tough, both physically, and emotionally/mentally. They have no problems taking risks and living on the edge. They can get by just fine by themselves. Others can be around, but they don't need anyone else.
ISFPs are not as reserved. While they don't like being the center of attention or in large groups, their close friends are important to them. They can be just as athletic as ISTPs or other SP types, and physically tough too, but they simultaneously have a softer side. They are concerned with presenting a consistent image in everything from their clothes, to car, to furniture, to musical taste. I think ISFPs are the most style-conscious of all the types.
Some physical stuff that isn't proof positive, but can be evidence for/against ...
- When playing sports, ISFPs can take a hit as well as ISTPs, but are far less likely to give one out. ISTPs combine grit and skill (pure grinders are more likely ISTJ), while ISFPs impress with agility, reflexes and awesome fine motor skills/hand-eye.
- ISTPs like machines, vehicles, and doing things with equipment (computers, cameras, shop equipment). ISFPs are more drawn to artsy stuff, like saxophones and art supplies.
- ISTPs dress in a down-to-earth and practical style, more like INTPs. ISFPs may dress to impress (not with brand names, but something that really reveals personal style). They often have a punk or alternative influenced look.
- Both types like cars and are very good drivers. But the ISTP is much more likely to get into the mechanical details of cars, and know how to grind breaks and tune engines. ISFPs are more concerned with how their car looks and aren't as likely to be expert mechanics. The ISTPs I know like tough, high horsepower cars like Camaros, Corvettes, and Challengers. ISFPs seem to like graceful and agile cars like Mini Coopers, Si's, and RX-8s. ISTPs shift for performance, while ISFPs want to be smooth.