With Boeing (BA 0.01%) expecting air traffic to double in the next two decades, the airline industry potentially represents a heap of profits. But knowing which company is set to benefit the most is difficult. After all, there is a lot of time between now and when the majority of those aircraft are paid for and in the sky.

Fortunately, several key indicators are already available that will help investors make an informed decision about which company will realize the most profits. It is important to note that it isn't just major airplane manufacturers that investors should consider, as several auxiliary firms could also be in for the ride. 

Inventory could be a sign of things to come
Boeing has increased inventory by 123% over the last three years. That's a good sign the company is gearing up for what could be busy years ahead. In the airline business, it is important to anticipate expected surges in demand, due to the length it takes to manufacture aircraft. Now that airlines are beginning to experience increased travel demand, by an expected 6% this year and 6.4% next year, Boeing is likely going to continue building inventory.

The expected surge will also provide airlines with the capital to purchase more aircraft in the years ahead, which utilizes Boeing's built-up inventory. In fact, WestJet announced on Aug. 29 the purchase of 65 MAX aircraft from Boeing.  

Parker Hannifin doesn't look ready to meet demand
As a manufacturer of the control and motion technologies, and systems involved in building aircraft, Parker Hannifin (PH -0.46%), might appear poised for profits. The firm has managed to regain double-digit operating margins from high demand in the firm's aerospace segment.

However, Parker could be ill-prepared to take on a surge in future demand. Inventories have fallen in each of the last three quarters, from $1.51 billion to $1.47 billion to $1.37 billion. While that is a great sign for short-term investors (the demand for products is on the rise), it shouldn't give long-term investors much confidence. After all, during the same period, revenue increased by 12%. Increased sales compared to lower inventory indicates the company might not be able to keep up with demand in future years. In addition to the quarterly inventory fall, annual inventory has also fallen off in each of the last three years ($1.41 billion to $1.40 billion to $1.37 billion). 

This could make shareholders cautious enough to rule the company out. However, investors might be able to forgive the rapidly decreasing inventory, due to the firm being "caught off-guard" by increased sales of its aerospace segment, according to Morningstar. This means the company could be able to increase its inventory now that it knows about the demand in its aerospace segment.

Honeywell looks ready for a boom
As a diversified aerospace technology and manufacturing firm valued at $62.4 billion, Honeywell (HON 0.38%) could be ready to take on a surge in the demand for its security, controls, and sensing products. Honeywell has raised inventory slightly in the last couple quarters, going from $4.2 billion in the fourth quarter of 2012 to $4.3 billion in the second quarter of 2013. This represents a 1.4% increase, while sales went up by 1.1% during the same period. This shows that the firm is able to handle the current volume of sales while also managing to increase its inventory in anticipation of a climb in demand. 

Expect the company to continue to pour money into building inventory, and spend less on acquisitions. The firm has traditionally focused its cash flow on expanding its business, (and this is a sign the company is in good health). Honeywell can now use the cash that it typically has on hand at the end of each quarter, to focus on inventory. Cash flow at the firm has traditionally been in the billions. At the end of last year, it was $3.5 billion, and $2.8 billion the year before.

Inventory is the common denominator
The inventory of companies in growing industries should play an important role in deciding which stocks to buy. Both Boeing and Honeywell look prepared for what's to come in aerospace, but Parker Hannifin appears caught off guard with the attention its aerospace segment has received. While I consider the other two companies buys, Parker Hannifin will need to prove it is able to increase inventories to meet aerospace demand before I consider picking up shares of this stock.