dealing with HuggingFace's model's tokens
For your first question, you can check if the tokenizer covers a certain string with the following:
text = 'today is a good day '
ids2string = lambda ids: tokenizer.convert_tokens_to_string(tokenizer.convert_ids_to_tokens(ids))
ids2string(tokenizer(text)['input_ids'])
&gt; &lt;s&gt;today is a good day &lt;/s&gt;

If emoji is not included in the tokenizer creation, the tokenizer will replace it with the unknown special token. You can access that with tokenizer.special_tokens_map['unk_token']. You can drop or keep them, shouldn't make much of a difference.
Alternatively, if you're going to fine-tune, you can add your own tokens to the existing tokenizer with tokenizer.add_special_tokens. However, in this case the embeddings for that token will be random. And you need to train them.