The easiest way is to look at the price tag. If it has less than 3 zeros before the period, it has MIM parts.

-Deke

Also look for parting lines as with any cast part. Slide stops have a faint line running lengthwise on the inside flat where they meet the slide. MIM parts don't have quite the heft a tool steel part does either, they feel fluffier.

One easy way to ID MIM parts is to look for tiny circular marks. These marks are caused by the pins that eject the part from the mold.

Virtually all handgun makers now use MIM parts to a greater or lesser degree.
S&W makes much use of MIM in their revolvers (hammer, trigger, and most other internal parts) and in their automatics.
The S&W MIM revolvers have firing pins inside the frame instead of on the hammer and the rear of the trigger has an hollow space on the back side that's easily seen.

Colt uses less MIM than just about anyone. They use MIM for the magazine catch, sear, and disconnecter. All other parts are forged steel or castings.

Ruger uses almost all cast parts.

Companies like Kimber make heavy use of MIM.

Used correctly and made right, MIM is perfectly suitable for some parts. As example Colt has used MIM sears and dis connectors for a good many years with no problems.
Where you see problems is in MIM parts not being made to high standards or used in areas that are not as well suited for it.
Too many companies trying to hold prices down to compete in the 1911 market don't do as good a job in MIM manufacturing and parts may not stand up. This is when you see broken parts.
Others are using MIM in parts that would be better made of castings or even forged parts but in trying to keep prices down they use MIM.

S&W recently stated that when made to the right standards MIM was as durable as forged steel and has proven to be so in their guns.

This is an age where cut-throat competition from foreign made guns, and more American makers means that we're going to see more and more plastic and MIM guns. That or prepare to spend unacceptable prices for guns others are selling much cheaper.
In the case of a 1911, you have the option of replacing the MIM parts with cast or forged steel.
In other guns, since there is no large industry offering upgraded replacement parts, you're limited to the gun as-is.

Besides the circles and parting lines, I've also noticed that the flats of some MIM parts look slightly concave.

Ejector pin marks or mold lines.