It depends on the construction in which it occurs.
He may BE guilty.
we know that 'be' is an infinitive, since only an infinitive has privilege of occurrence (i.e. the ability to be placed) directly after a modal auxiliary.
On the other hand, in
It is vital that she BE on time.
since an infinitive cannot occur directly after a nominative pronoun, we know that 'be' cannot here be an infinitive (it would actually, in this case, be a form of the subjunctive mood).
Note the subtle, but significant, difference between the terms 'infinitive' and 'base form': the latter is a description purely of the outward form of a verb, regardless of its functionality as a finite/nonfinite form. Thus both be's in the above sentences could legitimately be described as 'base forms', whereas only the first is a true infinitive.