if (strcmp("/back", cmdtext, true, 4) == 0)
    {
        TogglePlayerControllable(playerid,1);
        SendClientMessage(playerid,COLOR_PINK,"You are not AFK Anymore !");
        return 1;
    }
    if (strcmp("/afk", cmdtext, true, 3) == 0)
    {
        TogglePlayerControllable(playerid,0);
        SendClientMessage(playerid,COLOR_PINK,"You are now AFK!");
        return 1;
    }

new AFK[MAX_PLAYERS];

// ...

public OnGameModeInit()
{
    // Run a timer here
    SetTimer("CheckAFK", 1000, true);
}

// ...

public OnPlayerUpdate(playerid)
{
    // If the player just came back from being afk, the AFK variable is most likely more than 3/5 (vary this based on your gamemode and experience)
    if(AFK[playerid] > 3)
    {
         // This player just came back from being AFK
    }
    AFK[playerid] = 0;
}

// ...

forward CheckAFK();
public CheckAFK()
{
    // Suggestions for loop: either use foreach or at least define MAX_PLAYERS to your server's server.cfg amount!
    for(new i = 0; i != MAX_PLAYERS; i++)
    {
        if(!IsPlayerConnected(i)) continue;

        AFK[i] ++;
        if(AFK[i] == 3)
        {
            // The player most likely just went AFK!
        }
    }
}