There are two ways to tell, the price or the dimensions, both of which are listed on the Amazon pages. Typically a smaller book, which are called mass market paperbacks, are going to be between $6 or $10 dollars for a new copy direct from Amazon, while the bigger books, which are called trade paper backs, are usually around $15 or $20 dollars for a new copy direct from Amazon.
If you scroll down to the product details Amazon lists the dimensions, so if you're looking for the MMPB you'll want to go for the smaller version.
The dimensions are probably the better way to figure out which version you're looking at since most Star Trek MMPBs that were published more than a few years ago are sold by third party sellers, and their prices for the same book can literally range from a few dollars to hundreds of dollars.