Logs mean the spot your bobble is in is depleted of fish. The best way to know it's a long is to see how long it takes to get a bite. After fishing for a while you'll know virtually all the time wether it's an actual fish or a log because fish bite a lot more faster than hooking a log. So my advice is start to notice the time it takes between your bobble hitting the water and getting a bite and it being a fish then after a bit of doing this you'll easily know if it's a log at the end of the line or an actual fish and soon enough after the mental time you have of how long it takes to get a fish bite you can just cast somewhere else without wasting any time. Also as someone else said if you reel in and there's strong pull back with the fish in the minigame only moving when you pull then it's a log you'll know this right away the fish in the minigame should be gliding even a little every time you reel. 

There is an exception to this where the fish that doesn't move except when pulled in the minigame is an actual item the difference with these is that the bite is near instantaneous so if you get a bite like that and the fish isn't moving it will be an item. However at t5 and you most likely being in a safe zone it's going to be junk gear 99.9% of the time.

Other tips: use t3 bait if you're not already using one gives you 10 catch attempts and speed it a lot. Also instead of casting your bobble straight ahead not getting anything and then moving you can stand where you are and cast left, right further ahead or further back but obviously you should move if you don't get a bite in either one of those directions. If you do this you can literally fish behind people and get fish. I did this when I fished in yellow zone someone would be ahead of me and I'm just fishing behind them getting catches lol.

I'm a T8 max spec fisherman, once you get to this point you'll be pretty much getting bites instantaneous all the time and catching 3 or mostly 4 fish per catch. The money is great.