Signed or unsigned isn't a specific property of a byte or any , it's an interpretation. Unsigned -1 is 2^32-1, and stored as that. The bits of signed -1 are the same as the bits of signed 2^31+1, assuming it uses the normal two's complement metthod (
http://en.wikipedia.org/wiki/Two%27s_complement). It doesn't make a difference if Cheat Engine is interpreting everything as signed or unsigned, and because it isn't really possible (as far as I know) to tell what a program is interpreting a specific byte as from outside the program (I'm not sure about this, I think it's right, but please correct me if it isn't) it can't just use the program interpretation.
So if you're looking for a value you know should be negative, and Cheat Engine is interpreting everything as unsigned, search for greater than or equal to 2^31+1 (2147483649). If Cheat Engine is interpreting everything as signed, search for less than 0.