diff --git a/scripts/synthid-check.py b/scripts/synthid-check.py
new file mode 100755
index 0000000000..a9f371c648
--- /dev/null
+++ b/scripts/synthid-check.py
@@ -0,0 +1,79 @@
+#!/usr/bin/env python3
+'''
+  Checks if a text carries the SynthID watermark produced by llama.cpp with --synthid-keys.
+  The g-values are computed with the SynthIDTextWatermarkLogitsProcessor from HF transformers,
+  the score is the mean g-value: about 50% for unwatermarked text, higher for watermarked text.
+
+  Syntax:
+    ./scripts/synthid-check.py -f output.txt --keys 654,400,836 --tokenizer <HF model id or path>
+
+  The tokenizer must be the one of the model that generated the text.
+  The text should contain only the generated part, without the prompt.
+'''
+
+import argparse
+import math
+import sys
+
+import torch
+from transformers import AutoTokenizer
+from transformers.generation.logits_process import SynthIDTextWatermarkLogitsProcessor
+
+
+def score_tokens(token_ids, keys, ngram_len=5, sampling_table_size=65536, sampling_table_seed=0, context_history_size=1024):
+    processor = SynthIDTextWatermarkLogitsProcessor(
+        ngram_len=ngram_len,
+        keys=keys,
+        sampling_table_size=sampling_table_size,
+        sampling_table_seed=sampling_table_seed,
+        context_history_size=context_history_size,
+        device=torch.device("cpu"),
+    )
+
+    ids = torch.tensor([token_ids], dtype=torch.long)
+    g_values = processor.compute_g_values(ids)                       # (1, n_ngrams, depth)
+    mask = processor.compute_context_repetition_mask(ids).float()    # (1, n_ngrams)
+
+    depth = g_values.shape[-1]
+    n_scored = int(mask.sum().item())
+    if n_scored == 0:
+        raise ValueError("text is too short to score")
+
+    mean = (g_values.float() * mask[..., None]).sum().item() / (depth * n_scored)
+
+    # unwatermarked text gives i.i.d. Bernoulli(0.5) g-values
+    z_score = (mean - 0.5) / (0.5 / math.sqrt(depth * n_scored))
+
+    return mean, z_score, n_scored
+
+
+def main():
+    parser = argparse.ArgumentParser(description="check a text for the SynthID watermark")
+    parser.add_argument("-f", "--file", required=True, help="text file to check")
+    parser.add_argument("--keys", required=True, help="comma-separated watermarking keys, same as --synthid-keys")
+    parser.add_argument("--tokenizer", required=True, help="HF model id or local path of the tokenizer")
+    parser.add_argument("--ngram-len", type=int, default=5)
+    args = parser.parse_args()
+
+    keys = [int(k) for k in args.keys.split(",") if k.strip()]
+    if not keys:
+        sys.exit("error: --keys requires at least one key")
+
+    with open(args.file, encoding="utf-8") as f:
+        text = f.read()
+
+    tokenizer = AutoTokenizer.from_pretrained(args.tokenizer)
+    token_ids = tokenizer.encode(text, add_special_tokens=False)
+
+    if len(token_ids) < args.ngram_len:
+        sys.exit(f"error: text has {len(token_ids)} tokens, need at least {args.ngram_len}")
+
+    mean, z_score, n_scored = score_tokens(token_ids, keys, ngram_len=args.ngram_len)
+
+    print(f"tokens: {len(token_ids)}, scored ngrams: {n_scored}")
+    print(f"watermark score: {100.0 * mean:.1f}% (unwatermarked text scores about 50%)")
+    print(f"z-score: {z_score:.2f}")
+
+
+if __name__ == "__main__":
+    main()