How to check if a website page is prerendering?

The HubSpot developer tools include a debug option that will test if the page is prerendering

You can access the prerendering debug tool by adding this parameter to your URL:

?hsDebugOnly=true

Power Pro theme is designed to prerender, the results of the debug might display errors, but these warnings don't impact the performance.

Items that might impact prerendering include:

• Select HubL variables
• Personalization tokens
• HubL "today" function
• HubL "unixtimestamp" function
• Password-protected pages
• Pages that have adaptive tests
• Pages that include smart content (supported with partial pre-rendering)

An example of a HubL variable listed in the developer documentation that could cause partial pre-rendering is:

 {{ year }} 

This is commonly used in the footer to update the copyright date and when used in this way doesn't impact prerendering (tested and confirmed).