programming - How to distinguish between lists and values?

I have a (hopefully small) problem with some numerical integration algorithm, more specifically I want to integrate the imaginary part of a complex valued function, e.g. f[u_]:=Exp[-iuK] with $K\in\mathbb{R}$. As mentioned I am only interested in Im[f], in the example -Sin[u K].

Now if I integrate with Mathematica

NIntegrate[f, {s, Min[roots[[ 1 ]], roots[[ 2 ]]],
                Max[roots[[ 1 ]], roots[[ 2 ]]]}, AccuracyGoal->aGoal, 
                PrecisionGoal->pGoal, WorkingPrecision->wPrecision ];

I get two different results depending on f:

1. if I use -Sin[u K], it returns somenumber


2. if I use Im[f], it returns a list { somenumber } Those two have to be treated differently and that crashes my program. I have a few questions:

Why does Mathematica sometimes return lists, and sometimes values? How can I distinguish between a list and a value, i.e.

If xyz is a list then 
  do something 
else 
  do something else
end

Any other ideas how one could avoid these different return "types"? The manual and anything I found hasn't been useful so far.

Answer

How about putting the results in a list and then removing unneeded braces.?

a = 76.5
b = {4, 5, 12.3}
Flatten[{a}]
Flatten[{b}]

That way you are always dealing with a list of values.

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Or simply place braces around numbers (that are not already in a list):

If[NumericQ[x], x = {x}]