Learn to distinguish between the various eras that characterized the band's restless musical growth. The turbo-charged Clash of 1976-77 was not the rootsier animal unveiled on its first (1979) U.S. tour, nor the experimental, dub-inflected force that powered the extended European and Australian residencies in the autumn of 1981. Once you learn the types of songs that were played, you're less likely to fall for the common scams of multiple titles assigned to the same concert, or set-list orders changed to create a previously unreleased show.
Immerse yourself in the Clash's concert history. Although not hardcore jammers in the Grateful Dead tradition, the band made a point of changing set lists and arrangements. Most of this information is available on websites like Black Market Clash (see the link in Resources) or in bootleg rating guides--such as the Hot Wacks series--where fans' recollections and press clippings are also shared. These resources should help you cross-check multiple titles and listings to determine if you need a particular bootleg, or if it duplicates an existing release that already occupies your shelf.
Join Internet forums where you can connect with other fans and collectors to debate the merits of particular bootleg releases, live concerts and the band members' musical capabilities. Connecting with other fans who are as passionate as you about the Clash's music only widens the exchange of information, and provides an additional line of defense against getting scammed.
If you're new to the cause, focus on a particular era by arming yourself with 10 to 20 "starter" shows--since most bootlegs tend to be live concerts, anyway. Compact-disc recordings are the most prevalent format, effectively replacing cassette tapes. Vinyl bootlegs display numerous variations and tend to be scarcer, being pressed in limited quantities of 500 to 5,000 copies.
Revisit a particular site or forum to determine which bootlegs interest you, then seek out the collectors who are most likely to have them. Observe basic collecting etiquette: Introduce your request by letter or email, with a brief description of what you might offer in return. If you don't have a show to swap, ask for a B&P trade, in which you agree to cover the costs of blank discs and postage. Most collectors are happy to do this for new fans.
If your schedule and wallet permit, take some time to attend record conventions--such as the one hosted by WFMU, New York's own legendary free-form station. However, realize that most people who travel this circuit tend to impose higher markups for their wares, and are often less inclined to bargain (although deals are possible for the enterprising rookie collector). Scams also pop up more readily in these circles; the higher-priced the item, the greater the scrutiny that should be attached to it.
Scan eBay listings to seek out scarce or out-of-print titles. However, as with record conventions, use judgment before spending money too freely. Check with other collectors to determine the value of a certain title--many sellers aren't above manufacturing the odd "collectible" or two if it serves their purposes. For example, this author recalls seeing a poster touting the band's summer 1981 appearance at a New York City club--when they appeared two months earlier, during their now-famous 17-night Bond's Casino residency.