Differentiating between Elementary and Non Elementary reaction
Reactions that are probably not elementary

Whenever there are more than three reactants, it is unlikely that the reaction is elementary. Tenary elementary reactions are already quite rare. If you have a reaction like

$$\ce{2C8H18 + 25 O2 -&gt; 16 CO2 + 18 H2O}$$

it is impossible for 27 particles to come together at once, so you would expect some intermediates. Even for a reaction with just 3 reactant molecules such as

$$\ce{CH4 + 2O2 -&gt; CO2 + 2H2O}$$

it is highly unlikely that a single collision would be sufficient to break 6 bonds and make 6 new ones.

Conversely, a reaction with a single product such as

$$\ce{O + O2 -&gt; O3}$$

is probably not bimolecular because there is no way of getting rid of the excess energy. Instead, the reaction proceeds like this:

$$\ce{O + O2 + X -&gt; O3 + X}$$

where X is "a third molecule" that leaves the collision chemically unchanged but with higher kinetic energy (source: https://cimss.ssec.wisc.edu/wxwise/ozone/OZONE2.html).

Reactions where you can't tell

Bimolecular simple reactions could well be elementary, but often are not. The reaction

$$\ce{H2 + I2 -&gt; 2 HI}$$

shows two distinct mechanisms, one with one elementary step as written (the iodine has to be vibrationally excited, favored by high temperature) and the other one with iodine first dissociating into iodine atoms (source: https://link.springer.com/chapter/10.1007/978-3-642-80299-7_12).

Is it possible to tell just by looking whether a given is reaction is elementary or non elementary?

No, but sometimes it is obvious that the reaction has to be multi-step. In cases where a mechanism is dependent not only on the net reaction, but also on reaction conditions, it becomes obvious that just knowing the net reaction is not sufficient.