It's important to note the time of year you are looking at structure.
For example, fish as a general rule, particularly bass, in the very begining of the season when the water is cold, your looking to find more vertical structure in deeper pockets of water on main lake points that lead back into good creek channels.
As you look at a topo map, take your time, find the more vertical lines that are out on the main lake points, follow those points and see if they lead back to some shallow flats, shallow flats can be as deep as 10 feet or less, but the fish will let you know what they consider to be shallow, fish will use these flats to spawn, if you already know where the fish spawn in your lake, you are already ahead of the game, you can locate this area on your map and follow this creek out to a main lake point, in between those two areas will be a migration route, this migration route will be used by them on the way in and on the way out as well, this time of year they are scattered everywhere, they are moving up and down old creek beds looking for good spots to grab some snacks.
Things to keep in mind as you are fishing structure this time of year.
Deeper water, roughly around the 20 foot mark give or take a few feet, if you find a piece of structure, as for example a hump they are relating to, since you don't have a sonar, pay particular attention to detail, how far up the hump does your bait come before you get a strike, you may count your bait down and the water will be approx. 20 feet but the bait gets hit at approx. 10 feet, this could be a real good indication that you have a thermocline, and by looking at your map, this means you can eliminate any structure below that 10 foot mark, that is to say if the fish are giving you a consistant bite in the same area time and again, boat position is just as critical, make sure to pay really close attention to everything, remember you have no feedback from a sonar so you are going to be your own provider for information.
A couple of things that may help,
Pressured days or bright sunny days the fish will be tight to cover, these days are the toughest days, if you find yourself fishing in shallower waters, concentrate your efforts on heavy matted grass, thick lilly pads, docks, things of this nature will provide the necessary cover the fish will hang to, take your time and fish these areas thoroughly, and I mean pic them apart inch by inch, your best bet may be in much deeper waters on grass beds or in deeper shadowed areas.
Non pressured days or slighly over cast days the fish will tend to loosen up and expand from that cover and be more apt to chase a bait, so your presentations can speed up as well, but always experiment and let the fish guide you to what they want, fish will also generally make a move to shallower structure and cover under these conditions as well.
Bottom structure, the fish are mostly related to softer structure this time of year, as a general rule, softer bottoms are what you want to try and locate during the summer months with all of the above mentioned baits, this could mean anything soft like grass for example, during the transitions, generally speaking the harder bottom becomes more productive during the spring and fall seasons.
All of the rigs in the above posts will help you determine just what kind of structure you are fishing, without a sonar, it's tough and I hope that this helps a little.
Good luck and be safe !!!
