“I can tell you put a lot of work into it.”
Your friend is an artist, and he created a beautiful piece of art. It's very detailed, and you know by looking at it that it took a long time. You want to tell your friend that you can see all his hard work, so you say this.
I can tell you put a lot of work into it.
When you "can tell" something, it means that you are able to know it by seeing it, tasting it, feeling it, etc.:
I can tell that you're getting better at it.
Could you tell I wanted to leave?
You can leave "that" out in casual conversation, but in formal speaking or in writing you should definitely include it:
I can tell that you put a lot of work into it.
When you "put work into" something, it means that you work hard on it. You use the phrase "put work into ___" when you're talking about projects that need a lot of work, and take a while to finish. For example, you might "put a lot of work into":
- a big presentation for work
- an essay for school
- a website that you built
The adjective phrase "a lot of" is commonly used with "put work into ___". For example:
The Patzers bought their house for $ 125,000 in the early 1990s and put a lot of work into it, including some indoor remodeling.