Hi Smith,
I'm fairly sure you can not tell the oligomeric state of your protein from an mtz and your sequence.
What you can do is calculate the Matthews co-efficient with your given unit cell parameters and size of monomer to see how many copies of your monomer you may have in the a.s.u.
The difficulty lies in the fact that if say you have 4 copies of your protein in the a.s.u - this doesn't necessarily mean you have a tetramer - they could be 4 monomers related by n.c.s - or 2 dimers....
Is it possible to solve your structure? Then you confirm your space group and number of molecules in the a.s.u. - then look at the area of contact between monomers to see if they are likely to be because of multimerisation or crystal packing.
Totally agree with Tim that a better way (which you'd probably want to do to back-up a multimeric crystal structure) is by SEC or native PAGE. You could also try SAXS, SEC-MALS, EM, or DLS depending on the size of protein you're looking at. You'd also want to know that you have a clean oligomer and not an aggregate ;)
Good luck,
val