RegionMember

RegionMember[reg, {x, y, ...}] gives True if the point is inside the region.

Examples

Test point in disk:

RegionMember[Disk[], {0.5, 0.5}]
(* True *)

RegionMember[Disk[], {2, 0}]
(* False *)

Membership function:

mf = RegionMember[Ball[]];
mf[{0.5, 0.5, 0.5}]
(* True *)

Please visit the official Wolfram Language Reference for more details.