[Jefff]

pawn Код:

IsPlayerOnline(const nick[])
{
    if(!nick[0]) return INVALID_PLAYER_ID; // empty nick

    static name[MAX_PLAYER_NAME + 1];
    for(new i, g = GetMaxPlayers(); i < g; i++)
        if(IsPlayerConnected(i))
        {
            GetPlayerName(i, name, sizeof(name));
            if(!strcmp(nick, name))
                return i;
        }

    return INVALID_PLAYER_ID;
}

// in cmd
new ID = IsPlayerOnline(targetname);
if(ID != INVALID_PLAYER_ID) // or if you don't want ID use if(IsPlayerOnline(targetname) != INVALID_PLAYER_ID)
{
    // player is online
}
else
{
    // player is offline
}