Is there a word for something hidden in pants/trousers pocket but its shape is visible?

Solution 1:

People often refer to the state you describe as one invovling bulging pockets. Here is a google image search of "pocket bulge" (but beware, the search also returns risque images of underpants).

Urban Dictionary even has an entry for "back pocket bulge," although they restrict it to the bulge caused by an overstuffed wallet.

Bulge means

Swell or protrude to an unnatural or incongruous extent (ODO)

To describe the situation where a pocket bulge reveals information that someone is trying to keep secret, you might use the word betray, which means

Unintentionally reveal; be evidence of (ODO)

An example sentence would be:

"His bulging pocket betrayed that he had his phone."

You could also call such a bulge a telling pocket bulge, where telling means

revealing (Dictionary.com)

An example sentence would be:

"He had a conspicuous and telling pocket bulge."

Solution 2:

I could tell that he had his cell in his back pocket by its outline.

The outline is just another word for contour, a hard solid object such as a cell, a bunch of keys or a man's wallet will produce a distinctive shape in a fairly tight-fitting pocket.

outline (Cambridge Dictionaries)
the main shape or edge of something, without any details:
Underneath the hospital blankets I could see the outline of her poor wasted body.

There's even a question about outlines on LifeHacks Stack Exchange
How to prevent outline marks on trousers?

There's also the term fading as reported on Levis Strauss.com

The ‘Stuff-in-Your-Pockets’ Fading: While the wallet fade has become as ubiquitous as lap fades and whiskering, a newer popular pocket accessory—the iPhone—has launched scores of message boards online both in celebration and condemnation of cellphone outlines being worn on to the front and back of jeans.

You could combine both words as this person did on Pinterest

Love it when a man's jeans have the fade outline of a wallet.

Solution 3:

There isn't a word specifically for being able to see the shape of an object in a pocket, but the word silhouette means

the outline or general shape of something

One way to use it could be:

Don't try to claim you forgot your wallet and can't pay; I can see the silhouette in your pocket!

Silhouette implies that you can see the outline of the object's shape very well and can discern what it is. You shouldn't use it to describe when you can tell someone has stuff in their pockets, but you have no idea what. For example, you wouldn't say:

I know you have something in your pocket, because I can see the silhouette.

In that case, bulge would be more appropriate.