You have to be very careful about which certification to go for.
The CWI exam is widely recognized, however, if you intend to inspect welds and high strength bolting for steel buildings, some building departments (especially on the west coast) require that you be ICC certified....and some even require that you obtain BOTH certifications.
As weldtek pointed out, AWS does offer exam preparation seminars. It is important though, not just to rely solely on that 5 day seminar to get you through...because they will NOT supply all the answers during the seminar. My advice is that you get all the books and sample questions ahead of time...study for about 2 months...then take the seminar and the exam right after it.
For more information o the CWI exam, click the link below
Currently ICC has an exam (structural steel and welding special inspection) which covers welding and bolting inspection which will soon be discontinued. A few months ago, they introduced an exam for bolting inspection and a separate exam for welding inspection. These two new exams were developed to replace the structural steel and welding exam.
Although the AWS CWI exam is different from the ICC exam, IMO, they compliment one another. The CWI exam tests candidates on amongst other things: fundamental welding knowledge, ability to evaluate welds (they use plastic replicas in the exam), code knowledge (you chose 1 code for the exam from a list provided). There are 3 parts to the CWI exam, namely: code, fundamentals, and practical. One has to attain at least 70% in all 3 parts and satisfy work and education requirements to become a CWI.
For more information on the new ICC exams check page 30 of 53 in the pdf file