How can define an indicator which measures the degree of similarity between two signals?
The general topic of finding similarities between signals is wide ranging:


are the signals of same sampling, length, offset, shift or scale? 
where do they take their values (discrete, real, complex)?
are they stationary? noisy? 
what do you consider similar (whole signals, chunks, specific features)?
which are the invariances looked for?
and most important: what is your goal?


The following works provide an overview of some common similarity metrics, which one can normalize into an index:


Clustering of time series data --- a survey, section 2.2. Similarity/distance measures
Similarity Measures and Dimensionality Reduction Techniques for Time Series Data Mining, section 3. Similarity measures