Obviously, your usual "tender breasts" does not apply as a symptom of pregnancy when you are breastfeeding, though there are likely to be changes in your milk supply, and the taste of your milk may also change, due to hormonal fluctuations. Perhaps a child who fusses at the breast is one of the most obvious symptoms of your new pregnancy. Fussiness can imply a decrease in your supply, as well as a change in taste. Your breasts may also feel overly sensitive, and nursing might be slightly painful. Some women find themselves feeling strange and fidgety when they are feeding their older child, and might develop a sudden aversion to nursing.
I think this also qualifies as a definitely pregnancy symptom! If you conceived while still breastfeeding an older child, congratulations! You made it this far, and you do not need to stop now, if your milk supply has not dried up (as it does, for some women). There are women who successfully breastfeed throughout their pregnancies and then go on to tandem nurse their new baby and their older child. If you feel that you can no longer handle breastfeeding, and you feel aggressive while doing it, that is a valid sign to stop nursing though. Breastfeeding is beneficial as long as both parties are enjoying it. If nursing while pregnant does not feel right for you, or your supply decreased so much that your child does not want to nurse any more, don't feel guilty!