Look for teeny little bugs. Aphids and other plant sucking insects secrete a substance called honeydew, which is clear and sticky. If it's aphids, you can just wash them (and the honeydew) off with a soapy water spray (soap, not detergent) and then rinse your tree after that. If it is scale insects (which just look like little bumps) secreting the honeydew, you will have to touch them with a swab with rubbing alcohol. They have a shell which makes them impervious to the soapy water.
https://www.natureloveyou.sg/Plant%20Sto ... id%202.jpg
aphids come in various colors...
https://whatcom.wsu.edu/ag/comhort/nooks ... /scale.jpg
this is close up. In real life, they wouldn't be quite as obvious and there might not be so many of them.