Tagging fish is not illegal, but the Pennsylvania Fish and Boat Commission highly discourages the practice. According to the commission, tagging done by even the most experienced of anglers can cause problems for the fish. Depending on the type of tag used, the process can result in the skin or body cavity being punctured. This can lead to infection and even a slow death. Tagging also can disrupt normal behavior including feeding, swimming and even hiding.
Trout are easy compared to bass.
For trout, you can look at and try to remember the orientation of spots and related marks. Like snowflakes, no two trout will be alike. Keep a detailed log of where you catch fish, the time of day, the weather conditions, the water temperature and your fishing method. At the end of the season, you'll have a nice record of your activities to look back on and you'll likely discover some patterns that could make you a more productive angler.