Catch the event you want and check if cs_get_user_mapzones for the player has the corresponding bit for bomb target.
Code:
#define CS_MAPZONE_BUY (1<<0)
#define CS_MAPZONE_BOMBTARGET (1<<1)
#define CS_MAPZONE_HOSTAGE_RESCUE (1<<2)
#define CS_MAPZONE_ESCAPE (1<<3)
#define CS_MAPZONE_VIP_SAFETY (1<<4)
/* Returns in bitwise form if the user is in a specific map zone.
* NOTE: If user can't plant (cs_get_user_plant(index) is 0) then cs_get_user_mapzones(index) & CS_MAPZONE_BOMBTARGET will return 0 too.
*/
native cs_get_user_mapzones(index);
However, looking at the NOTE, it seems you would be able to just check cs_get_user_plant.
If you need to know for a player without the ability to plant, you could attempt to find the bounds of the bomb target's entity (pev_mins and pev_maxs) and check if the player is within that space.