[XStormiest]

First thing first, markers are reprsented by MapIcons, SetPlayerMapIcon search this function and learn it.
Secondly, for the verificaiton part, you need a function that is called IsPlayerInRangeOfPoint, search and learn it.
Thirdly, I will show you an example of using both of them.

Код:

public OnPlayerConnect(playerid)
{ 
   SetPlayerMapIcon(playerid, 0, yourxhere, youryhere, yourzhere, 10, yourcolorhere,  3);
   return 1;
}

Also an example for usage of IsPlayerInRangeOfPoint

Код:

public OnPlayerCommandText(playerid, cmdtext[])
{
    if(!strcmp(cmdtext, "/test", false))
    {
        if(!IsPlayerInRangeOfPoint(playerid, yourmaxradiussizehere, yourxhere, youryhere, yourzhere))
        {
            return SendClientMessage(playerid, yourcolorhere, yourmessagehere)
         }
    }
    return 0;
}