What are some tips to tell if you are close to checkmate?
It is extremely difficult to know how far you are from checkmate. If you could calculate as deep as you wanted (essentially, solve chess), then you would be able to look it up on a database. My suggestion then is that you try to calculate and see if you find a series of moves that do the job. As this series of moves will very likely proceed from a tactical motif, I strongly suggest that you study a few of the many indicators that allow you to know whether a tactical combination is possible.
when the opponent's king is open and you have a lot of pieces around your opponent's kings. Also, if your opponent does not have many defenders.
Checkmate can occur in thousands of different ways, usual as well as unusual. There are many typical checkmate patterns that you can learn by studying games, solving tactical problems, etc.
You cannot be "close" to checkmate. Either you have a combination that leads to checkmate or you don't.