Booking a holiday is / can be a major purchase and the last thing you want is for the holiday of a lifetime to turn into a nightmare. This can definitely be avoided by doing just a little detective work.

Is the travel company you are looking at registered with any governing bodies, such as ABTA, ASTA, ATOL etc

Does the company accept payment by Credit Card - This is HUGELY important important as if a company accepts payment by credit card (not including credit cards via paypal) then they will be a) more tracable and more importantly you will have greater rights whould you have problems on your trip and wish to make a claim against the company.  

What contact details are on offer. If a travel company is genuine and trustworthy, then you would expect to have registered office addresses, phone numbers etc and NOT JUST an email address.

Can you find independent reviews? 1 or 2 complaints online is only to be expected. Honestly, NO company can keep everyone happy and sometimes there will be clients who will just not be happy, no matter what. But if you find page after page of negative reviews, then there probably is no smoke without fire.

Are the company an agent or a tour company. Generally you will have less come back if the company you are booking with are an agent only, so where ever possible, see if the company is a tour operator and if not, have a long hard think about your potential booking !