Resist the urge to high-five everyone in the cubicles next to you. Your stock may have just strapped on a rocket pack and taken off for the moon, but smart investors won't celebrate until they know that upward leap was justified. Without a fundamental basis for the bounce, these stocks can quickly make the return trip down.

Is now the time to lock in profits, or is this just the first step toward even higher valuations down the road? Let's examine several stocks that just hit the afterburners and see whether they're truly headed into orbit.

Stock

CAPS Rating (out of 5)

Monday's Change

Youku.com (Nasdaq: YOKU)

*

30.0%

LinkedIn (Nasdaq: LNKD)

*

12.0%

Westport Innovations (Nasdaq: WPRT)

*

11.6%

Believing Greece will approve the necessary austerity measures for a European Union bailout, the markets continued rising, jumping another 145 points, or 1.2%. Yet there's rioting in the streets of Athens and the situation remains precarious, so stocks that went significantly higher are pretty big deals.

Waiting for the last minute
It's easy to ridicule the social-media bubble and its Chinese counterpart. Seems for every Facebook there's a Renren, and for every Twitter there's SINA's Weibo.

The copycatting doesn't end there, with Youku.com angling to be China's YouTube. But it got more than a "me too" vote of confidence yesterday after inking a deal with Time Warner (NYSE: TWX) to offer U.S. video-on-demand titles to Chinese Internet users. The three-year deal with Warner Bros.' entertainment division began beta testing last October, and since then it's generated 200,000 paid transactions.

Still, investors aren't convinced Youku will be such a big hit. Less than half of the CAPS members rating the video service think it can beat the market, but only one in five All-Stars agrees.

Share your opinion on the Youku.com CAPS page and follow its video exploits by adding the carrier to the Fool's free portfolio tracker.

Remember this
The social-media set got a further boost when several analysts upgraded their opinions of LinkedIn, the professional-networking site often considered the "grown-up's Facebook." From JPMorgan Chase (NYSE: JPM) and UBS (NYSE: UBS) to Morgan Stanley and Bank of America, the analyst community thinks the business model could be a "$10 billion long-term revenue opportunity" and the company a $92-per-share stock. That's a pretty audacious claim, considering the share price has cratered since it hit $100 a stub after the IPO.

Before you get swept up in the hubbub, realize that these analysts might just be protecting their base. They were the underwriters in LinkedIn's IPO; other analysts were a bit more sanguine in their assessment. Evercore Partners thought LinkedIn was worth an "equal weight" rating, whatever that really means.

Considering that the excitement over Facebook may be wearing thin as it has shed some 6 million users, how long will it be before the novelty of LinkedIn wears off? To CAPS All-Star Lordrobot, "[It's] a bubble though it probably has a place in the new social software cloud base ... sort of dot comish at these valuations."

Stay connected to LinkedIn's developments by adding it to your watchlist to keep tabs on who's chatting up the stock.

Climbing out of the ditch
There was no social-networking connection to Westport Innovation's jump yesterday. Its good news had to do with a deal it struck with General Motors to bring its natural-gas technology to the light-duty-vehicle market. Clean Energy Fuels (Nasdaq: CLNE), which is building a nationwide network of natural-gas fueling stations, also got a bump from the news.

Oil prices have been beaten back lately, but they're still at elevated levels, giving renewed impetus to alternative fuel solutions. The problem is that deals such as Westport's are for long-term change, not anything immediate. GM won't have a fleet of Westport-inspired nat-gas cars hitting the roads anytime soon. It proves, though, the viability of the technology and perhaps of the value of an investment in the alt-fuels technology leader.

CAPS member flacobob calls the natural-gas alternative a "no brainer," and that view probably aligns with the thoughts of the more than 97% of CAPS members rating it to outperform the broad market indexes. It's also my best-performing pick on CAPS. Add Westport Innovations to the Fool's free portfolio tracker to see whether it can fuel additional growth deals.

Going into orbit
That's why it pays to start your own research on these stocks on Motley Fool CAPS, where you can read a company's financial reports, scrutinize key data and charts, and examine the comments your fellow investors have made, all from the stock's CAPS page. Then you can decide for yourself whether your stock's headed for re-entry or off to infinity and beyond.

The Motley Fool owns shares of JPMorgan Chase and Bank of America and has opened a short position on Bank of America. Motley Fool newsletter services have recommended buying shares of SINA, Westport Innovations, and General Motors. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. 

Fool contributor Rich Duprey has no financial position in any of the stocks mentioned in this article. You can see his holdings. The Motley Fool has a disclosure policy.