How do I Tell if I am in the Mun’s SOI

[Little Kerbonaut!]

So I was trying to send my SSTO to the Mun. I did all the maneuvers you had to do to get into a Mun encounter, but when it came to trying to get sucked into the Mun’s SOI I failed to do it. So does anyone know where the Mun’s SOI is I think it’s when you touch the “Mun encounter marker” but I don’t know. Anyone who does know please reply.

[overkill13]

Yes, when you pass the Mun Encounter Marker, you will enter the Mun's SOI.  In order to remain there, you are going to need to slow down, otherwise you will exit the Mun's SOI again.  You need to burn retrograde at some point, preferably your periapsis around the Mun until you enter a stable orbit.  You won't just get "sucked in" because you are traveling too fast and the Mun doesn't have enough mass.

[Gargamel]

The wiki will have the exact numbers of the size of the SOI, but it honestly doesn't really matter.    When you are planning your intercept burn, as you fiddly with it, you will see your orbital path change color at the point of entry into the new SOI.   For bodies in a bad position or too far away to easily see from the ship in the map view, cycle the Tab key until you focus on the body you want.  You should still be able to manipulate the node from there.   That way you can fine tune your approach. 

[Little Kerbonaut!]

Oh ok, thanks for telling me.