The first step is to do some research and learn about the basics of baseball card grading and values. Look up websites like Beckett, MLB.com, or PSA/DNA which provide information on card prices and historical sales data. See what certain rare/valuable cards have sold for to get an idea of potential value. Familiarize yourself with common grading scales like the 1-10 scale used by PSA/DNA.

Carefully examine the condition and grade of each card. Things like centering (how perfectly centered the image is on the card), corners (are they sharp or rounded/damaged), edges (are they frayed or pristine), and surfaces/printing (any flaws, scratches, stains etc.) all impact a card’s grade and price. Higher graded cards in mint condition are nearly always worth more. Take your time inspecting under good lighting.

Consider the player featured on the card. Iconic all-time great players tend to have the most valuable cards, especially if they are from their rookie seasons. Even cards of lesser known players could hold value depending on other factors. Research players’ careers and accomplishments to get an idea of demand.

Note the year, brand/manufacturer and any special markings. Older, vintage cards from the 1950s-1980s generally command higher prices due to their scarcer supply over time. But rookie cards, error variations, special logo/sticker cards and others from any era could be quite valuable too. Things like oddball brands/sets may be collector favorites.

Find out print runs/rarity when possible. Lower print run cards are scarcer and more desireable to collectors. Serialized or autographed rare parallel issue cards can also be very valuable depending on specific details.

Check for special signings, autographs or relic cards featuring game-worn memorabilia. Autographed cards and those with actual uniform swatches, bat/ball pieces are prized and gain exponentially in worth especially from star players. Even basic relic cards still carry a premium.

Pay attention to the state of the card stock/paper. Fragile, discolored or degraded paper can hurt grades/value over time. Keep cards in protective sleeves, holders or a binder if valuable. Properly stored condition is key.

After consideration of all factors, compare recently sold prices online through researching auction sites like eBay. Consider using services like PSA/DNA to have valuable cards professionally graded which can further authenticate and validate condition/value. Ungraded, raw cards may sell for less than similarly graded counterparts.

With patience and diligence, you can get a good idea if your baseball cards hold any potential value worth pursuing through more formal appraisal or sale. Having large collections also increases odds of finding that one rare, valuable card that makes the entire group tremendously valuable.