How can I find out if a student’s thesis is just the translation from his native language (Spanish) to English? One of my student’s MA thesis is very well done. I know him and I worked with him enough to notice that something is going on. I talked to him and he always seemed confused about what he was doing and he did not seem to know the source so well. 

Is it possible that he just translated and paraphrased another thesis from his native language, which is Spanish? How can I find it out? 

The thesis contains citations and everything but it’s too accurate and I have some doubts.
Your question suggests that you are not sure which of the following is true:


Your student worked hard and produced an excellent thesis
Your student plagiarised a thesis from another language


In either case, this warrants extra attention from you. 

The first step would be to discuss the work with him further. If he was able to discuss said work intelligently, this would be an indication that he did write the thesis. However, you said that you spoke to him, and he seemed confused about what he was doing, which raises legitimate suspicions. 

To find out whether he can (and has) produced good written work, but finds it hard to discuss said work in person, you can ask him to edit his written work. Unless the thesis is 100% perfect in every way, you can identify a potential area of improvement that should not place an undue burden on the student. For example, you can ask him/her to


add some extra detail to one section
compare to another related work in the literature review section
etc.


If the student did the work and independently produced a great thesis, this is positive attention that will help improve the work. If the student did not do the work, it will become evident when he tries to improve it following your suggestions. 

Of course, if you know someone who speaks the student's language and who can be counted on to keep your request quiet, you can (without sharing the student's name) ask him/her to search the web in the student's language for publicly available theses on the subject, and see if a match turns up.