Concerns about U.S. debt and a Greece bailout remain at the forefront of investors' minds, but it wasn't enough to derail dozens of companies from hitting new 52-week highs this week. For optimists, these rallies may seem like a dream come true. For skeptics like me, they're opportunities to see whether companies trading near their 52-week highs have actually earned their current valuations.

Keep in mind that some companies deserve their lofty valuations. Secondary education company and Rising Star pick Bridgepoint Education (NYSE: BPI) is one such company. With enrollment growth that puts rivals Apollo Group (Nasdaq: APOL) and Corinthian Colleges (Nasdaq: COCO) to shame, and earnings results that continue to crush Wall Street's estimates, it's no surprise to see the stock trading at a new 52-week high.

Still, some other companies might deserve a kick in the pants. Here's a look at three companies that could be worth selling.

Utopian economics
Miller Energy Resources
(NYSE: MILL) continues to work its slight-of-hand tricks on paper, but it's not fooling me!

This explorer, producer, and driller of oil and natural gas purchased the assets of Pacific Energy Alaska Operating LLC in bankruptcy court for $2.25 million in 2009, then immediately turned around and claimed a $277 million gain on its purchase, adjusting for what it deemed to be underpriced assets. Since the purchase and subsequent one-time gain Miller's stock has nearly tripled -- but is this warranted? I'd say no.

The company hasn't produced anything near a full-year operating profit yet based on its drilling and exploration activities. Revenue figures show huge growth, but considering that the company was essentially starting from zero, those results need to be taken with a grain of salt. Valued at more than 12.5 times sales and still potentially years from turning an operating profit, Miller's going to need to entice me with tangible results rather than rabbit-out-of-the-hat tricks to get me on board.

Going flat
Everybody is buzzing about fizzy drinks these days. Three weeks ago, Hansen Natural was my target, last week it was SodaStream, and this week it's National Beverage (Nasdaq: FIZZ).

On paper, things look like they're heading in the right direction. National Beverage does have a healthy balance sheet with no debt. Profitable, and trading at 16 times forward earnings, what could possibly be wrong? I suggest you take a closer look at its growth projections.

That's not a misprint, folks: expected revenue growth of 1%-2% per year over the next two years. There's only so much frosting you can put on the cake before the lack of real substance becomes apparent. Over the past year, revenue is up 1.1% thanks in large part to a 1.2% increase in unit prices. This recent trend of price increases outpacing sales growth is a disturbing trend and speaks volumes to the lack of organic growth at National Beverage. If shareholders aren't careful, their investment could wind up flat.

Wait before you bet the farm
While it's difficult to value a pharmaceutical upstart based solely on its results, sometimes we can't ignore them.

Infinity Pharmaceuticals (Nasdaq: INFI) has moved steadily higher to a valuation of $240 million, yet none of its five drug candidates have made it past phase 2 trials. Data has so far been encouraging, but despite earning revenue derived from its alliance with Purdue and Mundipharma, Infinity has no marketable products.

This isn't the first time we've seen a biotech company take off with all of its drugs still early in clinical trials -- and it won't be the last. Investors need to focus on not letting their emotions get the best of them and trade based on the facts. Right now, the facts show a company that's scheduled to run out of cash in 2014 with a steady stream of losses. Investors would be smart to wait for late-stage phase 3 trials to consider an investment here.

Foolish roundup
Jumping the gun before the results are in is one of the most common investing mistakes. Whether it's a biotech, oil company, or beverage maker, waiting for signs of tangible revenue growth and profits are the keys to a successful investing plan.

What's your take on these companies? Share your thoughts in the comments section below and consider adding Miller Energy Resources, National Beverage and Infinity Pharmaceuticals to your watchlist to keep up on each stocks' respective sector.

Fool contributor Sean Williams has no material interest in any companies mentioned in this article. You can follow him on CAPS under the screen name TMFUltraLong The Motley Fool owns shares of Bridgepoint Education. Motley Fool newsletter services have recommended buying shares of Hansen Natural and SodaStream, as well as creating a call spread position in Bridgepoint Education. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy that never needs to be sold short.