Algae grows in water, not fuel. So, no water, no algae.
Having said that though....
Fuel sits atop water. That means IF there is water in your tank, it sits at the bottom. Right where the end of the pick up tube for your lift pump is.
So....water in tank means that water AND any "bugs" (algae) in that water will be immediately pumped to your engine.
How does water get in the tank in the first place?
Well, first off, ALL pump fuel has a bit of water in it. So....you get a little to maybe some more, anytime you fuel up depending on the place you pump from. Not a big deal really because you have at least two filters and, often, a water trap.
But...when you let it sit for years, the water can actually increase due to condensation from the atmosphere as your tank "breathes" through the vent system.
So....
You end up with the water you started with plus accumulated condensation.
That's what determines how much algae you may have grown over time.
Long haul trucks can travel 1 million miles with little to no algae problems. But, for example, boats that mostly sit at the dock with a fuel load can grow copius amounts of algae in their tanks.
It's bad because the condensate moisture tends to cling to the tank walls and algae will not only grow in the water at the bottom of the tank but also on any exposed surfaces inside the tank.
As a result, a tank that is half full will grow more algae over the long storage time than one that is full of fuel.
That's why you may have heard the term "tank polishing". It's often required to clean/scrub the algae from the tank walls.
Old fuel can be "polished" too by running it through filters at elevated temperatures to clean it and remove the moisture. It's expensive though and usually only done in very large batches.
So, as someone else has already said, don't end up spending $thousands$ to save hundreds.
If you're lucky, your tank came with a drain plug. You could choose to drain some if so and have a look at what you might be up against.
Siphoning will NOT give you the same information unless you can figure out how to siphon directly from the tank's sump.
And, neither method will tell how much growth has taken place on the tank's walls.
Good luck.