[cracker]

How can one tell if the bones are of a mammal or not?

The ear bones.

The location of the fossils in the earth, the age and nature of the surrounding fossils and geologic formations, previous experience with fossils that are similar to these, etc. Years of training and experience in the field. Research done by hundreds of other professionals who have reached similar conclusions. The total absence of any argument that the fossils are not mammals (what else would they be, and where is the evidence for that?).

[VadeRetro]

The location of the fossils in the earth, the age and nature of the surrounding fossils and geologic formations, previous experience with fossils that are similar to these, etc.

We might as well mention the lower jaw bone as well. The lower jaw of mammals is a single, seamless bone.

[gore3000]

"The total absence of any argument that the fossils are not mammals (what else would they be, and where is the evidence for that?).

Surmissions, suppositions, and extrapolation are not facts. Bats can fly, yet they are not birds. Whales can swim, yet they are mammals. These and many other animals "break the mold" set for them. Surmissions, suppositions and extrapolations are a procrustean bed which not only proves nothing, but in fact is inimical to true science. Science relies on facts, not wishful thinking.