new Float:LastPosHash[MAX_PLAYERS];
new InactiveSeconds[MAX_PLAYERS];

public OnGameModeInit()
{
    SetTimer("UpdateTimer", 1000, 1);
    return 1;
}

stock GetPositionHash(playerid)
{
    new Float:ppx, Float:ppy, Float:ppz;
    GetPlayerPos(playerid, ppx, ppy, ppz);
    return floatround(ppx * ppy * ppz / 3);
}

forward UpdateTimer();
public UpdateTimer()
{
    for(new i = 0; i < MAX_PLAYERS; i++)
    {
        if(IsPlayerConnected(i) && !IsPlayerNPC(i))
        {
            if(LastPosHash[i] == GetPositionHash(i))
            {
               InactiveSeconds[i]++;
            }
            else
            {
                InactiveSeconds[i] = 0;
                LastPosHash[i] = GetPositionHash(i);
            }
        }
    }
}

stock IsPlayerInactive(playerid)
{
    if(InactiveSeconds[playerid] > 10) return true;
    return false;
}

/*Usage Example:

CMD:inactive(playerid)
{
    if(IsPlayerInactive(playerid))
    {
        SendClientMessage(playerid, -1, "You are inactive!");
    }
    else
    {
        SendClientMessage(playerid, -1, "You are not inactive!");
    }
    return 1;
}