How can I send message as Text Message on purpose?
There is currently no way to switch to Text Message by default when iMessage you know is going to fail. 

1) When you know that your iMessage is not gonna go then try this:


Type iMessage and send it. It will get stuck with Top bar showing "Sending..."
Long press the iMessage text that is not going.
A menu will appear where you can choose "Send as text message"


2) Now if you want to send Text Message intentionally by passing iMessage (consider scenario when you know recipient is not on cellular data or wifi) you can do the following:
- Disable your Wifi
- Disable Cellular data ( Go to Settings -> Select "Cellular" -> disable "Cellular Data"
- Type iMessage and send it. It will get stuck with Top bar showing "Sending..."
- Long press the iMessage text that is not going.
- A menu will appear where you can choose "Send as text message"

Hope that helps!

Screenshots attached:Updating after fsb's comment below. "Send as text message" is not disabled in iOS 10.2.1 (latest as of 02/21/2017)