You have a few options to check if a player is in a part:
- GetPartBoundsInRadius - https://create.roblox.com/docs/reference/engine/classes/WorldRoot#GetPartBoundsInBox
- GetPartsBoundsInBox - https://create.roblox.com/docs/reference/engine/classes/WorldRoot#GetPartBoundsInBox
- GetPartsInParts - https://create.roblox.com/docs/reference/engine/classes/WorldRoot#GetPartsInPart
But if you want to make your life easier, you can use ZonePlus:
ZonePlus is very easy to setup and there are many features, such as:
-PlayerEntered
-PlayerExited
-LocalPlayerEntered
-LocalPlayerExited
-ItemEntered
-ItemExited
And much more.
I recomend using ZonePlus