Some examples:

Everything must be known at the time of evaluation:

?- 2*2 =:= X.
ERROR: Arguments are not sufficiently instantiated

?- X=4.0, 2*2 =:= X.
X = 4.0.

?- cos(X) =\= sin(X).
ERROR: Arguments are not sufficiently instantiated

% Ok, then!

?- X=pi, cos(X) =\= sin(X).
X = pi.

Sadly =:= is a bit fussy. It shouldn't even worry about X here:

?- X*0+1 =:= 1.0.
ERROR: Arguments are not sufficiently instantiated

However it can deal with subexpressions:

?- X=(1+1), X*2 =:= 4.0.
X = 1+1.

"is" can do that too:

?- X=(1+1), Y is X*2.
X = 1+1,
Y = 4.

Differences in arithmetic type (https://eu.swi-prolog.org/pldoc/man?section=artypes) don't matter.

?- 4 =:= 4.
true.

?- 4 =:= 4.0.
true.

This works even for rational numbers (https://www.swi-prolog.org/pldoc/man?section=rational):

?- X=1/7, X =:= 2/14.
X = 1/7.

Evaluation occurs on both sides:

?- 1*10 =:= 2+3+5.
true.

?- sqrt(2.0) =:= sqrt(4.0/2)
true.

The constant "epsilon" is the machine epsilon (https://en.wikipedia.org/wiki/Machine_epsilon) and reveals floating point limitations:

?- 1.0 =:= 1.0 + 0.5*epsilon.
true.