What type of slides - do they mount to the sides of the drawers or underneath the drawers? The ones that mount to the sides are easy to get tilted; those that mount underneath have 50% less chance of being tilted since only the slide pieces mounted to the cabinet itself are more susceptible to tilted mounting.
See if the slides are parallel to the floor: open the drawers while looking from the side of the case... see if the drawers open uphill or downhill. Or, with the cabinet sitting level, puit a ball/marble into the drawers and see if it consistently rolls in one direction.
For most drawer slide units, there are elongated holes in each piece - both horizontal and vertical ones besides smaller round holes. Ideally you use only the vertical holes for either case or drawer components and the horizontal holes for the other component. This way you can adjust the up/down of the drawer by using the vertical slots, and the in-out with the horizontal slots. I typically use the vertical slots of the slide pieces mounted to the drawers, referencing the leading edge of the slide to the leading/front edge of the drawer side panels. The horizontal slots get used on the slide parts mounted to the cabinet so I can adjust the in-out travel. Using spacer blocks nearly as long as the drawer slides (just scrap wood cut on the table saw to a consistent size) makes it easy to hold the cabinet mounted parts parallel to the cabinet floor: stand the spacer on the cabinet floor and rest the slide piece on the spacer while marking/drilling the pilot holes and then installing just 2 screws for now. I make a second spacer to position the slide a constant distance from the face frame too. Use the same spacers on the other side of the cabinet and the slide pieces ought to be installed identically. Make a third spacer that will sit on the installed slide pieces to get the spacing for the next cabinet mounted slide, etc. Some folks use a single large scrap piece of sheet material (plywood, etc) and make one tall spacer and install the TOP drawer slides first, then they cut off a few inches for the next slide down, etc. Either way works fine. My way does allow errors to accumulate though; the single sheet method chops up a bigger piece of wood but errors don't accumulate.
You can make a spacer for the drawer mounted slide pieces too - for times when the slide pieces are mounted to the sides (not bottom) of the drawer. Mark a line where the center of the drawer slide should be... then clamp the slide piece on that centerline. Make a spacer that goes from the bottom edge of the drawer to the bottom edge of the slide... then screw another scrap to the bottom of the spacer making a "L" shape. Now it self-registers against the drawer bottom and is easy to clamp in place so you can concentrate on holding the slide piece, drilling, etc. Install only 2 screws initially.
Once the drawers are adjusted and everything is lined up, remove the drawers (separate the slide halves - most will come apart again) and use the small round holes this time... use up the remaining screws to finish the installation.
Since most drawer slide assemblies have way more holes than screws you can easily correct an "oops" by picking different holes... and just ignoring the small screw hole left in the cabinet walls or drawer sides. Or fill it with glue+sawdust.