The Bible is God speaking, so checking what Scripture says about things and making sure we are in alignment with His ways is one solid way to know what to pray for and not pray for, and when it is something we want versus something God wants.
In addition to this is to listen with our hearts. God is love (1 John 4:8) and when we are in alignment with His will, which is always loving to Him, us, and others, He promises to fulfill those desires in His Word (Psalm 37:4). God doesn't speak in the ear, and loud booming voices don't interject when we need guidance or pray for insight into His wishes.
If you've ever felt the strong urge to do something uncomfortable and help someone while knowing full well it could end in personal embarrassment or just feel like it would be a waste of time, then you've heard God speaking to your heart. He lives in us (1 Corinthians 3:16) and when He speaks we feel. Now bear with me, not all "feelings" are God speaking, but He does move us to compassion and puts it in our heart to speak up about Christ to people and do the loving thing if we are listening to Him (with our heart).
You have to get "tuned in" to Him, be focused on Him, and know what He says in the Letter He wrote to us called the Bible. I have yet to date not found answers to my questions in that book, and it isn't the book but the fact I'm searching and listening to Him that rewards me for seeking His guidance (Matthew 7:7=Promise). It doesn't matter what we are reading, because He isn't limited to what He wrote down, but if we are reading we are listening to Him, and focused on Him, and He rewards us for that and those sought after insights start popping into our heads and the picture begins to clear up and our path becomes obvious (Hebrews 11:6).
My experience is the more I let go of concerns and focus on Him, the faster I obtain the clarity I was too busy seeking previously to focus on Him.