what is the key forensic signs that a drowning may not be a suicide,but a murder made to look like suicide?is petechial hemorrhages on the lungs...

Petechia on the lungs are more consistent with
suffocation, particularly when petechia are also present on the mouth or pharyngeal
tissues; finding these would tend to indicate the possibility of
murder.

Pathologists look for a white foamy discharge in
the airway as a confirmatory sign of drowning; generally they also look for water
aspirated into the lungs, which is carefully examined for the presence of diatoms, which
are tiny single-celled algae that are present in virtually all natural surface waters.
Interestingly, the specific blend of diatoms can be used as a "fingerprint" to link the
aspirated sample back to a certain body of water.

So
overall, petechia and lack of foamy discharge and/or water in the lungs would point to a
conclusion of dry suffocation. Finding water in the lungs that does not match the water
the body is located in (for instance chlorinated drinking water in the lungs, body in a
pond) would indicate that the body was drowned in one location and then moved to
another.

Another consideration would be to see if the body
has other external injuries consistent with struggling or being held under
water.

I am attaching a link to an article about this that
you may want to have a look at.