My way is all about converting this expression to a equivalent transfer function and let you see the frequency peaks in Bode plot. The following code does the trick:
syms t
x=sin(2*pi*10*t)+sin(2*pi*40*t)+sin(2*pi*50*t)+sin(2*pi*60*t);
Xs=laplace(x);
XsTf=syms2tf(Xs);
opts=bodeoptions;
opts.FreqUnits='Hz';
bode(XsTf,opts)
The first two line defines the x expression symbolically. Then, I took the laplace transform of the expression and the converted it to transfer function(the attached syms2tf function does it). Afterwards, since I obtain a transfer function expression, I expect to see peaks at those frequencies in Bode Plot and the attached figure contains it. Hope this helps to you.
