Question: How to find specific indeterminates in a expression?


I need to find all the variants (I am not sure if this is the correct term to be used but I hope this will be clear in the example) of a specific indeterminate in a given expression.  Here is an example:

alpha[3, 5]*xi[1]*xi[8] + alpha[3, 5]*xi[4]*xi[5] + alpha[3, 3]

For this particular example, xi[1], xi[8], xi[4], and xi[5] are the variables I am looking for. The indexes of xi change depending on the previous calculation.  Also, in some cases, I need the alpha variables instead, that is, alpha[3,5], and alpha[3,3].  

I could not figure out how to use indets in this case.

Many thanks for your help. 

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