Typically in-room dining is charged to the room. The person who delivers it will hand you a slip of paper to sign. There will be room on this slip to add a tip. You don't know if that tip will go entirely to the person delivering your food, or be pooled with other people in the same department, but tipping that way is the norm.
If you prefer, you could leave that line blank and hand cash to the person delivering, I'm sure it would not be refused.
If the hotel adds a 10% or 15% charge for in-room dining, the menu will tell you so. This technically means you don't need to tip, but it's considered a "required" tip anyway. I usually add another 15% on the slip. In room dining is expensive, and if adding a tip to it makes unrealistic for you, then best to go out and get some food.