Answer:
One has the accent - yes. Without the accent, it means, "if".
Step-by-step explanation:
You can generally understand this through contextual clues. If there is a question being asked, typically the yes is followed by a comma.
For example:
Tienes amigos? - Sí, tengo muchos. (Yes, I have many (a lot)).
*If you have a reason or another situation being posed after it, then it is likely the if meaning. Typically, there would not be a comma after it, rather a clause of some sort.
Example: Tienes amigos? -- Si me preguntas porque crees que no soy popular, entonces, no te voy a contestar. (if you are asking because you think I'm not popular, then I am not going to answer you).
Or perhaps a conditional phrase if you have learned it...
Si tuviera más oportunidades para viajar.... (If I'd had more opportunities to travel...)