You can plot it over a large range to see the trend
x = linspace(-1000, 1000);
y = @(x) 1./(1+10.^(x/400));
plot(x, y(x))
You can also put the value at infinity
>> y(inf)
ans =
0
>> y(-inf)
ans =
1
If you have symbolic toolbox
syms x
y = 1/(1+10^(x/400));
fplot(y, [-1000 1000])
And find the limits
>> limit(y, x, inf)
ans =
0
>> limit(y, x, -inf)
ans =
1
