It's going to be hard to tell by just external apperances. Both more or less respond the same way when approached. ISFJ's and INFJ's are really really similar when it comes to interacting with friends and everyone else. This is because of Secondary Fe. They also both appear to be adversed to any kind of sudden or sharp change, because of Ne and Se being inferior resectivly. Telling apart Si and Ni is really easy to do on paper. In person, this is actually really tricky. Both are internal processes, and you have to pull out the persons internal processes to tell. This is hard for ISFJ's because they are notoriously bad at explaining how they feel and think inside. In some cases, they will tell you it is impossible, and they arent lying either. INFJ's are similar. They often have a hard time explaining how the feel and think inside. The tell tale sign though, for me at least, is if you are able to get the person talking. INFJ's once given the right level of coaxing are able to open up slightly, and explain things to you. In some cases they wont stop doing so.
Both are also really passive and adversive to conflict, so both will try to mediate situations that have gone bad. The difference is how they do it. ISFJ's will do it in such a way to try to make the other people around you look worse then you. INFJ's will try to make you feel better about yourselfs. Both will try to reason what the other person has done wrong, but INFJ's tend to be much more sympathetic about both sides.
This is also not easy for people to do, but INFJ's and ISFJ's actually have pretty different "feels" to them. That feeling you just get by viewing a photograph.
Hope this helps.