The easiest test is dimensional. 18650 is the dimension of an unprotected cell--18mm in diameter, 65.0mm long. Please measure length with a plastic caliper, or something that won't short them! If they are protected (unlikely if coming from a laptop), they'll still be around 18mm long, but be a little longer (maybe 68.0mm or so).
Next question: voltage. A DMM can tell you the voltage right now: an 18650 lithium ion should be between 2.8 and 4.2 volts. Under 2.8, and they have been over-discharged (or are some other chemistry); over 4.2, and they are some other chemistry. A cheap 18650 flashlight can give you a basic voltage check (light or no light); some flashlights give voltage readings, as well (4 clicks on a Zebralight; 3 clicks on an Emisar D4).
A battery charger can tell you voltage and recharge them. More advanced models can tell you the capacity that they have "given" to the cell, and can do a discharge test to see how much capacity it can hold. Internal resistance can be an indicator of cell health--there are testers out there built into some chargers, and online instruction about how to do it with a DMM and a resistor.
More info about chargers, batteries, and other related topics than you ever wanted to know is at lygte-info.dk, including some information about marking of different cells, in case you want to really tell the lineage of your cells based on marking, color, dimensions, etc. Harvesting cells from laptops and neighbors' Teslas is fun, but I might start by researching a charger/tester at lygte-info to keep yourself safe and allow you to sort the stinkers out from the useful cells. For me, that sort is by capacity and internal resistance.
Best of luck!