I don't think it's easy to confuse them.
As for how to spot an ISFP, I have only met one ISFP in real life. Pretty sure she is an ISFP because she is so similar yet so different from me.
Similarities: The first time I met her, we have an instant friendship click with each other. There is no awkward silence or everything, everything feels really comfortable, like I have met a kindred spirit. We would go into a deep conversation with each other and tell each other about our personal lives, and we both tend to dislike things like small talks
Differences: She is more gullible than me lol. When people tell her something, she believes their words immediately. When people tell me something, I always try to see whether they are being serious, joking, or lying. Her auxiliary Se also makes her very blunt and insensitive at times, which rubs me the wrong way sometimes since Se is in my shadow function