Hi Ali. I imagined two cases given that your question is a bit ambiguous. First, in the case you know the original signal X, two measures could give you an idea of the decomposition level of a signal x extracted from X : (1) signal length, and more general (2) frequency spectrum. Second, if you are given a signal, let's say z, and the question: What level of decomposition is z?, The answer would be none. It is not decomposed as far as you can tell because the question does not states if z was obtained from a big Z. However, if you know the nature of the phenomena that generated z, then (2) will give you the answer again. For example, if z was generated by measuring muscle electrical activity, called EMG: Electromyography, it is commonly sampled at 1 kHz. Then if the spectrum of z is confined inside a bandwidth of ~125 Hz then you could say it was decomposed twice: (level zero bandwidth: 500 Hz, level 1: 250 Hz, level 2: 125 Hz...).
Decomposition level can be identified by looking at the frequency spectrum .Wavelet decomposition for instance is like a filter bank applied to your signal in which each level will represent a corresponding frequency.