Does this happen after you shift, or JRA. As far as cassettes go, it doesn't take much to make a small cog slip. The chain wrap on small cogs is poor and they wear faster than the larger cogs. I tried a number of ways to figure this out buying used freewheels. Best way is to take a new chain and lay it over 1/2 of the cog and pull the chain tight. The chain pins should bottom out in the cogs. If it appears the chain needs to be stretched out more for the pins to bottom out then the cog is worn. Another way is to try and find an identical cassette (a good photo will do) and compare the tooth profiles of the smaller cogs. Unless you are familiar with all the profiles or have a something to compare it to (some start out looking shark finned) it is difficult tell if the profile has changed due to wear.
Best way may be to try and make the chain skip. Start off in your large chain ring and shift from cog to cog, putting a lot of torque on the pedals. If the cassette starts skipping with one or more cogs, you'll know where your problems lie. BTW, you drivetrain must be moving to make a cog skip, it won't work if your stationary.