You would be evaluating the company just like any other individual stock, which means looking at its revenue, profit, debt, cash on hand, and other financial metrics. As well as its business model, and whether that business has any growth potential. But at the end of the day, the price is driven purely by how much other people want the stock in rhe market. I don't mean to discourage you from Learning about it if it interests you, but there are a ton of financial planners and analysts who make this their career, and are still wrong often.

If I were in your shoes, I would put in as much as you think you can afford over the term of the ESPP buy-in, and then simply plan to sell them all once as soon as you can. They will take all that money, and buy shares with it at the discounted price, and then when they hit your account, they will be "worth" the current price. The price you sell them at will differ a small amount from the price they were issued at, because the market will have moved in that time. The difference between the discounted price and the current price will show up on your W2 as income, and you will have to pay tax on that, which may eat into your refund little bit. But not only is that difference free money for you (after taxes), but you get back all of what you paid in, making it a form of forced saving. And it may be enough of a bundle of cash that if you still wanted to invest you could then buy an index fund which will still go up and down with the market but has less risk if any individual company underperforms.

Edit: just realized you said you were in the EU, I think all the ESPP rules are similar, but possibly in Metric instead. :)