The way to tell if you have internal parasites is to test the stool. There are many reasons that the stool can different than normal. Knowing normal for abnormal stool can be a great key to telling if something might be starting to go wrong in the first place. What was already alluded to above, if you need to know why this happened to your gecko you will need to have you vet run a battery of test which can get up there in cost. best of luck to the two of you.

Tom