As others have said... he's had time to study the stones.. but, more importantly, things that HOUSE the stones.
He's been able to study the cube that contains a stone
He's been able to study the "glow stick of destiny" (Loki's staff) that holds a stone
He was probably studying Vision to see how he could hold a stone in his head
While Tony used to be an engineer, in the latest movies they've been showing him as a Data Scientist. He's created neural nets / AI's that grind though vast amounts of machine learning / deep learning. The way this works is you set up inputs, nodes and an output / goal. The nodes (could be thousands.. millions) are all decision points. Then you fire off the inputs (what you know about the stones, what they've been in, how they react to things.. eg: some stones seem ok to hold by hand by a human, like the soul stone, while others consume the user unless they're sufficiently powerful enough) you send all that through the machine learning / deep learning algorithms and then yuo.. wait... until it spits out an answer that best matches your goal.
The trick with machine learning / deep learning algorithms is that they're shooting for statistical probabilities. You want models that are 100% accurate. But, sometimes you only get 90% or less.. predicitive modelling, etc. (eg: in data sci they had us run genetics from known cancer patients and mash up against unknown patients. You create a training data used to train your model, and a test data set to test it to see how accurate it is. For really complex stuff, you're going to be at less then 100% accuracy / prediction.)
Tony gets baked at the end of the movie, which means his gauntlet was flawed.. it worked to do the task, but didn't protect him from the blowback of power. But, it protected him amazingly well seeing as how it let a mere human use all 6 tones to accomplish greatness and still be alive after-the-fact to live long enough to know they won. He was there and saw how Thanos got burned from the backlash of the gauntlet. So, he probably knew that he had to factor in the power of the stones. But, when time is a factor, you can only run so many models for so long before you have to go "you know what, it's not 100% fool-proof, but it's the best we got." If he had more time, he might have been able to make a guantlet that surpassed even the Dwarves. But, with limited technology and limited time.. he did the best he could with what he had. And, it was enough to enable a human being to weild all 6 stones and snap his fingers. It killed him, but that's still ludicriously impressive... seeing as how the power stone alone almost fried Quill in the first Guardians movie.. and he was half-cosmic.
tl;dr ... Tony is the ultimate geek in these movies, from engineer to data scientist, and he's got AI's and deep learning at his disposal.
If we can (in real life) have IBM's Watson beating the pants off of contestants on Jeopardy, then imagine the kind of AI tech a super-rich tech genius super-hero guy must have to help him invent toys.