Debugging encoding problems (R XML)

Is there a way to locate an encoding problem within an XML file? I'm trying to parse such a file (let's call it doc) with the XML library in R, but there seems to be a problem with the encoding.

xmlInternalTreeParse(doc, asText=TRUE)
Error: Document labelled UTF-16 but has UTF-8 content.
Error: Input is not proper UTF-8, indicate encoding!
Error: Premature end of data in tag ...

and a list of tags with presumably premature end of data follows. However, I'm pretty sure that no premature ends exist in this document.

Ok, so next try:

doc <- iconv(doc, to="UTF-8")
doc <- sub("utf-16", "utf-8", doc)
xmlInternalTreeParse(doc, asText=T)
Error: Premature end of data in tag...

and again a list of tags follows along with line numbers. I've checked the lines and I can't find any errors.

Another suspicion: the "µ"-character that occurs in the document might cause the error. So next try:

doc <- iconv(doc, to="UTF-8")
doc <- gsub("µ", "micro", doc)
doc <- sub("utf-16", "utf-8", doc)
xmlInternalTreeParse(doc, asText=T)
Error: Premature end of data in tag...

Any other suggestions for debugging?

EDIT: After having spent two days with trying to fix the error, I still haven't found a solution. However, I think I have narrowed down the possible answers. Here is what I've found:

copying the XML string from the source database into a file and saving it as a separate xml file in Notepad++ --> Document labelled UTF-16 but has UTF-8 content.
changing <?xml version="1.0" encoding="utf-16"?> to <?xml version="1.0" encoding="utf-8"?> (or encoding="latin1") within this file --> no error
reading XML string from database via doc <- sqlQuery(myconn, query.text, stringsAsFactors = FALSE); doc <- doc[1,1], manipulating it with str_sub(doc, 35, 36) <- "8" or str_sub(doc, 31, 36) <- "latin1" and then trying to parse it with xmlInternalTreeParse(doc) --> Premature end of data in tag...
reading the XML string from database as above and then trying to parse it with xmlInternalTreeParse(doc) --> Document labelled UTF-16 but has UTF-8 content. Input is not proper UTF-8, indicate encoding ! Bytes: 0xE4 0x64 0x2E 0x20 Premature end of data in tag... (list of tags follows).
reading the XML string from database as above and parsing with xmlInternalTreeParse(doc, encoding="latin1") --> Premature end of data in tag...
using doc <- iconv(doc[1,1], to="UTF-8") or to="latin1" before parsing doesn't change anything

I would appreciate any suggestions very much.

The encoding problem occurred because the encoding of the original XML file and the encoding within the SQL database where the XML content was stored as longtext didn't match. Substituting the specification of the encoding within the XML string and converting this string solved the problem:

doc <- sqlQuery(myconn, query.text, stringsAsFactors = FALSE)
doc <- iconv(doc[1,1], to="UTF-8")
doc <- sub("utf-16", "utf-8", doc)
doc <- xmlInternalTreeParse(doc, asText = TRUE)

Truncating of the XML string during retrieval from the database turned out to be a separate problem. The solution is provided here: How to retrieve a very long XML-string from an SQL database with R?