This is quite easy actually. All investments have an intrinsic value, meaning a true net worth. Real estate, stocks, bonds, commodities, businesses, everything has a true cost! The trick is how to find it.
For instance what is a replacement cost of a home? If you can find something for less than that it may be a great buy. Can you cover a home's expenses with the rental income? Again might be a good buy. If neither of these are met then you might be in a bubble.
Same goes for stocks. Are you buying a stock with a PE of 25 and the cash flow growth rate is only 5%? If so, you're paying way too much! What if there's no income? What if there areT no assets? This is how bubbles form and how you could potentially lose a lot of money. Just look at the tech bubble 13 years ago.
If you're looking to buy an investment and don't know what the true value of it is nor how to figure it, then don't do it! For instance, what's the true value of gold? Good luck figuring that out!