Differentiating Escort Cards for Entree Selection

Aside from making the cards themselves different colors, I don’t see what you could do besides stickers that would make it noticeable to the waitstaff. Most formal weddings I’ve been to have had small stickers in the shape of the animal of choice in the corner of the card.

Here are a few ideas. You could color code the cards (black for beef, white for chicken etc.) To match your color scheme.
I think the envelopes are cute too.
I dont think a sticker or some sort of marking is tacky.

Our venue mandates us to use B for beef, C for chicken, etc. I’m handwriting all the placecards in calligraphy. Personally I don’t think it’s tacky and not something I think guests will make a big deal over

We had 2 dinner options at our local reception. Because most chose beef we didn’t mark those place cards. For the few folks who ordered pasta, we put a red crystal sticker on their place card. Easy!

Stickers won't look tacky at all-- you can get them done with calligraphy or a typewriter or something that looks nice, it doesn't have to be generic label-maker font! I actually really like having the menu selection on the back of my place card, I like being able to put it in a scrapbook as another memento of the reception.

Colour coding or stickers are also an option, as others have said!

We had a formal wedding and did gold stickers on escort cards. We had a calligrapher write the names.

We used a straight line across the top, 1/8 the inch of stick on foil, in blue, gold, silver, etc for the top edge of place cards. With a second band of the same color for a special diet. Since we had a limited menu allowing 6 basic choices, variations of entree, people checked in at the wedding, and before going in to be seated, chose their meal. And the Inn staff clicked in into a laptop, and put a strip of foil on each place card. It is a little classier looking than picture stickers of fish and chicken I have seen! Having worked catering, it is so much faster, quieter, and pleasantly unobtrusive when staff have lists by table caterer has made if meals needed to set up on trays or carts before going to the dining room, and card labels that staff can see from 8-10 feet away. Just a flash of a color, so you approach each person with the right pre-ordered meal in hand, barely interrupting conversation. If done in advance, time for ink to dry, those liquid metallic ink gold, silver, bronze, and glittery black, are very quick and easy, a line across the top like part of a design. But otherwise I have always found the foil tape less like a kid's school project than stickers. I also like it when place cards are displayed on hooks on a chart, to take to the correct table and choose a seat, when a hole is punched in the top, and a loop of 1/16th inch or 1/4" cloth satin Offray ribbon in different colors by meal is used. Simple, classy looking, a single hole punched in the top center of the card.