If an object is at rest, can we conclude that no external forces are acting on it?

No.  That conclusion would be irresponsible, and is not supported
by any of the everyday, familiar laws of motion, which, by this stage
of our education, we can recite in our sleep.

When we observe an object at rest, any intelligent one among us should
say to himself: 

-- "Self !  That object is at rest ... at least in MY frame of reference. 

-- It is not speeding up, it is not slowing down, and the direction of
its motion is not changing.

-- In short, its velocity is not changing.

-- In even fewer words, its acceleration is zero.

-- I have learned that  F = m A .  Perhaps even cooler than that,
I remember what it means and how to use it !

-- In the case of that object at rest in front of me,  A = 0. 
That tells me that  F = 0 .

-- I remember that 'F' is the vector sum of all the forces
acting on the object.

So, the conclusion that I CAN draw regarding that object at rest, is:

-- There may or may not be external forces acting on it.  I have no way
to tell. 

-- But if there ARE any, then I know that they are balanced ... their vector
sum is zero, and they have the same effect on the object as if there were
no forces at all acting on it. 

-- The NET force on the object is zero.