The biggest advantage to me is just being able to learn areas well by getting out and hiking them in the off-season. Some water sources are dry on certain years so being able to confirm or predict that prior to the hunt is helpful. I look for all different routes in and out of places. Evidence of hunters from previous years such as flagging on branches, spent shells, arrows, carcasses of previous kills, candy wrappers, plastic water bottles, chew cans, etc. It's pretty easy to tell where people hunt every year by just hiking around. Then I'll go into an area I like the day before the opener and see where people are camped and get a feel for where they might end up in the morning and try to plan around that. I've had places that I want to be opening morning but the day before I hiked in and saw a tent set up nearby so I went somewhere else. It's important to me that I'm the first vehicle parked at the access point I've chosen if possible. It's not to deter anyone from parking next to me that morning or hunting the same area that day but I like to be up high early so I can watch for other headlamps coming in to see where they go.
That's for the areas I day hunt.
For areas that require packing camp in with or without pack animals, I try to go on at least one late summer backpacking trip to look around. I look for previous camps, outfitter camps, and all the same evidence of prior hunting activity. I've printed maps of outfitter camp locations provided by the USFS and generally try to avoid those locations or look for where their activity might push animals. The condition of trails is important year to year as some might not have been cleared from a blow down event or just from all of the beetle kill falling which can obviously make travel slow or impossible, especially if pack animals are coming. Some of this info can be obtained by making phone calls, some definitely cannot and that's where being able to go up on my day off and check things out is the most helpful.
To summarize, I mostly try to learn the land well, look for environmental changes from year to year, and observe human activity. Seeing animals and sign is always good too but it's pretty far down my list for pre-season scouting since that
changes so much once people show up.